Just a general question say the signature of a function is:
void f( const string & s )...
Why is it necessary to pass this by reference if you are not actually changing the string (since it is constant)?
There are two alternatives to passing by reference - passing by pointer, and passing by value.
Passing by pointer is similar to passing by reference: the same argument of "why pass a pointer if you do not want to modify it" could be made for it.
Passing by value requires making a copy. Copying a string is usually more expensive, because dynamic memory needs to be allocated and de-allocated under the cover. That is why it is often a better idea to pass a const reference / pointer than passing a copy that you are not planning to change.
When you pass a variable by value, it makes a copy. Passing by reference avoids that copy. You want to mark it const for the same reason: Since you're not making a copy, you don't want to accidentally mess with the original. I think this could also potentially allow for compiler optimizations.
The reason you don't usually see this for int, char, float, and other primitive types is that they're relatively cheap to copy, and in some cases, passing by reference is more expensive (for example, passing a char by reference could involve passing 64-bits of data (the pointer) instead of 8-bits. Passing by reference also adds some indirection, which isn't a big deal with a big type like a string, but is wasteful for something like an int.
It is not "necessary," but the common answer is "for performance reasons, to prevent copying," however that is a naive answer and the truth is a bit more complex.
In your example, assuming s really is immutable and something you don't "own or can't change," then the const decorator is appropriate for s. If the reference of s wasn't taken, then that would guarantee a copied (excluding compiler optimizations).
If f() is not going to use the copy of s after f() returns, then the effort of copying s was wasted. So, passing by reference prevents the copying and f() retains the ability to inspect the string s. Great. And again, that's the naive answer and pre-C++11, would be the mostly correct answer.
There are more scenarios worth considering in order to answer "is it necessary?" but I'll focus on just one:
If the caller of f() doesn't need the string s after invoking
f(), but f() needs to retain a copy of the data.
Suppose the code is:
void f(const std::string& arg1); // f()'s signature
void g(const std::string& arg1) {
std::string s(arg1);
s.append(" mutate s");
f(s);
}
In this case, you would have constructed the string s, passed it by const reference to f(), and everything is fine from a performance perspective if you assume f() is opaque and there are no further optimizations available.
Now, suppose f() needs a copy of the data in s, then what? Well, f() will call a copy constructor and copy s in to a local variable:
// Hypothetical f()
void f(const std::string& s) {
this->someString_ = s;
}
In this case, by the time the data is stored in someString_, the normal constructor will have been called in g(), and the copy ctor will have been called in f(), however the work done in g() will have been wasted. To improve performance, there are two things that can be done, pass by value and/or use move constructors.
// Explicitly move arg1 in to someString_
void f(std::string&& arg1) {
this->somestring_ = std::move(arg1);
}
void g(const std::string& arg1) {
std::string s(arg1);
s.append(" mutate s");
f(s);
}
Which is explicitly doing what the compiler will automatically do starting with C++11, which means the more correct version is to pass by value and let the compiler do the right thing:
void f(std::string arg1) {
this->somestring_ = arg1; // Implicit move, let the compiler do the right thing
}
void g(const std::string& arg1) {
std::string s(arg1);
s.append(" mutate s");
f(s);
}
And in this case, the string is constructed in g() and no additional work was done anywhere. So in this case, the answer to,
Why is it necessary to pass this by reference if you are not actually
changing the string (since it is constant)?
The string wasn't changed, but it was copied, and therefore const reference was not necessary.
It's an exercise for the reader to list the optimizations the compiler can take when s is or isn't mutated after the call to f().
I can't recommend enough that people look in to David Abrams's post, 'Want Speed? Pass by value' or Is it better in C++ to pass by value or pass by constant reference? and post How to pass objects to functions in C++?.
in other words, you would get the speed of passing by reference ( not making extra copies ).
and the integrity of passing by value ( the original variable value is not changed)
It isn't 'necessary', but it's a very good idea, for several reasons:
Efficiency. Not creating or destroying new strings is more efficient than creating and destroying them, especially as strings are arbitrary in length and therefore require dynamically allocated memory.
A string can be constructed from a string literal. If you specify const you allow the compiler to construct a temporary string from a literal so that the caller can just provide the literal rather than the string object. If you don't specify const the compiler can't do that, so the caller can't do that either.
Related
I'm learning C++ at the moment and try avoid picking up bad habits.
From what I understand, clang-tidy contains many "best practices" and I try to stick to them as best as possible (even though I don't necessarily understand why they are considered good yet), but I'm not sure if I understand what's recommended here.
I used this class from the tutorial:
class Creature
{
private:
std::string m_name;
public:
Creature(const std::string &name)
: m_name{name}
{
}
};
This leads to a suggestion from clang-tidy that I should pass by value instead of reference and use std::move.
If I do, I get the suggestion to make name a reference (to ensure it does not get copied every time) and the warning that std::move won't have any effect because name is a const so I should remove it.
The only way I don't get a warning is by removing const altogether:
Creature(std::string name)
: m_name{std::move(name)}
{
}
Which seems logical, as the only benefit of const was to prevent messing with the original string (which doesn't happen because I passed by value).
But I read on CPlusPlus.com:
Although note that -in the standard library- moving implies that the moved-from object is left in a valid but unspecified state. Which means that, after such an operation, the value of the moved-from object should only be destroyed or assigned a new value; accessing it otherwise yields an unspecified value.
Now imagine this code:
std::string nameString("Alex");
Creature c(nameString);
Because nameString gets passed by value, std::move will only invalidate name inside the constructor and not touch the original string. But what are the advantages of this? It seems like the content gets copied only once anyhow - if I pass by reference when I call m_name{name}, if I pass by value when I pass it (and then it gets moved). I understand that this is better than passing by value and not using std::move (because it gets copied twice).
So two questions:
Did I understand correctly what is happening here?
Is there any upside of using std::move over passing by reference and just calling m_name{name}?
/* (0) */
Creature(const std::string &name) : m_name{name} { }
A passed lvalue binds to name, then is copied into m_name.
A passed rvalue binds to name, then is copied into m_name.
/* (1) */
Creature(std::string name) : m_name{std::move(name)} { }
A passed lvalue is copied into name, then is moved into m_name.
A passed rvalue is moved into name, then is moved into m_name.
/* (2) */
Creature(const std::string &name) : m_name{name} { }
Creature(std::string &&rname) : m_name{std::move(rname)} { }
A passed lvalue binds to name, then is copied into m_name.
A passed rvalue binds to rname, then is moved into m_name.
As move operations are usually faster than copies, (1) is better than (0) if you pass a lot of temporaries. (2) is optimal in terms of copies/moves, but requires code repetition.
The code repetition can be avoided with perfect forwarding:
/* (3) */
template <typename T,
std::enable_if_t<
std::is_convertible_v<std::remove_cvref_t<T>, std::string>,
int> = 0
>
Creature(T&& name) : m_name{std::forward<T>(name)} { }
You might optionally want to constrain T in order to restrict the domain of types that this constructor can be instantiated with (as shown above). C++20 aims to simplify this with Concepts.
In C++17, prvalues are affected by guaranteed copy elision, which - when applicable - will reduce the number of copies/moves when passing arguments to functions.
Did I understand correctly what is happening here?
Yes.
Is there any upside of using std::move over passing by reference and just calling m_name{name}?
An easy to grasp function signature without any additional overloads. The signature immediately reveals that the argument will be copied - this saves callers from wondering whether a const std::string& reference might be stored as a data member, possibly becoming a dangling reference later on. And there is no need to overload on std::string&& name and const std::string& arguments to avoid unnecessary copies when rvalues are passed to the function. Passing an lvalue
std::string nameString("Alex");
Creature c(nameString);
to the function that takes its argument by value causes one copy and one move construction. Passing an rvalue to the same function
std::string nameString("Alex");
Creature c(std::move(nameString));
causes two move constructions. In contrast, when the function parameter is const std::string&, there will always be a copy, even when passing an rvalue argument. This is clearly an advantage as long as the argument type is cheap to move-construct (this is the case for std::string).
But there is a downside to consider: the reasoning doesn't work for functions that assign the function argument to another variable (instead of initializing it):
void setName(std::string name)
{
m_name = std::move(name);
}
will cause a deallocation of the resource that m_name refers to before it's reassigned. I recommend reading Item 41 in Effective Modern C++ and also this question.
How you pass is not the only variable here, what you pass makes the big difference between the two.
In C++, we have all kinds of value categories and this "idiom" exists for cases where you pass in an rvalue (such as "Alex-string-literal-that-constructs-temporary-std::string" or std::move(nameString)), which results in 0 copies of std::string being made (the type does not even have to be copy-constructible for rvalue arguments), and only uses std::string's move constructor.
Somewhat related Q&A.
There are several disadvantages of pass-by-value-and-move approach over pass-by-(rv)reference:
it causes 3 objects to be spawned instead of 2;
passing an object by value may lead to extra stack overhead, because even regular string class is typically at least 3 or 4 times larger than a pointer;
argument objects construction is going to be done on the caller side, causing code bloat;
I heard a recent talk by Herb Sutter who suggested that the reasons to pass std::vector and std::string by const & are largely gone. He suggested that writing a function such as the following is now preferable:
std::string do_something ( std::string inval )
{
std::string return_val;
// ... do stuff ...
return return_val;
}
I understand that the return_val will be an rvalue at the point the function returns and can therefore be returned using move semantics, which are very cheap. However, inval is still much larger than the size of a reference (which is usually implemented as a pointer). This is because a std::string has various components including a pointer into the heap and a member char[] for short string optimization. So it seems to me that passing by reference is still a good idea.
Can anyone explain why Herb might have said this?
The reason Herb said what he said is because of cases like this.
Let's say I have function A which calls function B, which calls function C. And A passes a string through B and into C. A does not know or care about C; all A knows about is B. That is, C is an implementation detail of B.
Let's say that A is defined as follows:
void A()
{
B("value");
}
If B and C take the string by const&, then it looks something like this:
void B(const std::string &str)
{
C(str);
}
void C(const std::string &str)
{
//Do something with `str`. Does not store it.
}
All well and good. You're just passing pointers around, no copying, no moving, everyone's happy. C takes a const& because it doesn't store the string. It simply uses it.
Now, I want to make one simple change: C needs to store the string somewhere.
void C(const std::string &str)
{
//Do something with `str`.
m_str = str;
}
Hello, copy constructor and potential memory allocation (ignore the Short String Optimization (SSO)). C++11's move semantics are supposed to make it possible to remove needless copy-constructing, right? And A passes a temporary; there's no reason why C should have to copy the data. It should just abscond with what was given to it.
Except it can't. Because it takes a const&.
If I change C to take its parameter by value, that just causes B to do the copy into that parameter; I gain nothing.
So if I had just passed str by value through all of the functions, relying on std::move to shuffle the data around, we wouldn't have this problem. If someone wants to hold on to it, they can. If they don't, oh well.
Is it more expensive? Yes; moving into a value is more expensive than using references. Is it less expensive than the copy? Not for small strings with SSO. Is it worth doing?
It depends on your use case. How much do you hate memory allocations?
Are the days of passing const std::string & as a parameter over?
No. Many people take this advice (including Dave Abrahams) beyond the domain it applies to, and simplify it to apply to all std::string parameters -- Always passing std::string by value is not a "best practice" for any and all arbitrary parameters and applications because the optimizations these talks/articles focus on apply only to a restricted set of cases.
If you're returning a value, mutating the parameter, or taking the value, then passing by value could save expensive copying and offer syntactical convenience.
As ever, passing by const reference saves much copying when you don't need a copy.
Now to the specific example:
However inval is still quite a lot larger than the size of a reference (which is usually implemented as a pointer). This is because a std::string has various components including a pointer into the heap and a member char[] for short string optimization. So it seems to me that passing by reference is still a good idea. Can anyone explain why Herb might have said this?
If stack size is a concern (and assuming this is not inlined/optimized), return_val + inval > return_val -- IOW, peak stack usage can be reduced by passing by value here (note: oversimplification of ABIs). Meanwhile, passing by const reference can disable the optimizations. The primary reason here is not to avoid stack growth, but to ensure the optimization can be performed where it is applicable.
The days of passing by const reference aren't over -- the rules just more complicated than they once were. If performance is important, you'll be wise to consider how you pass these types, based on the details you use in your implementations.
Short answer: NO! Long answer:
If you won't modify the string (treat is as read-only), pass it as const ref&.(the const ref& obviously needs to stay within scope while the function that uses it executes)
If you plan to modify it or you know it will get out of scope (threads), pass it as a value, don't copy the const ref& inside your function body.
There was a post on cpp-next.com called "Want speed, pass by value!". The TL;DR:
Guideline: Don’t copy your function arguments. Instead, pass them by value and let the compiler do the copying.
TRANSLATION of ^
Don’t copy your function arguments --- means: if you plan to modify the argument value by copying it to an internal variable, just use a value argument instead.
So, don't do this:
std::string function(const std::string& aString){
auto vString(aString);
vString.clear();
return vString;
}
do this:
std::string function(std::string aString){
aString.clear();
return aString;
}
When you need to modify the argument value in your function body.
You just need to be aware how you plan to use the argument in the function body. Read-only or NOT... and if it sticks within scope.
This highly depends on the compiler's implementation.
However, it also depends on what you use.
Lets consider next functions :
bool foo1( const std::string v )
{
return v.empty();
}
bool foo2( const std::string & v )
{
return v.empty();
}
These functions are implemented in a separate compilation unit in order to avoid inlining. Then :
1. If you pass a literal to these two functions, you will not see much difference in performances. In both cases, a string object has to be created
2. If you pass another std::string object, foo2 will outperform foo1, because foo1 will do a deep copy.
On my PC, using g++ 4.6.1, I got these results :
variable by reference: 1000000000 iterations -> time elapsed: 2.25912 sec
variable by value: 1000000000 iterations -> time elapsed: 27.2259 sec
literal by reference: 100000000 iterations -> time elapsed: 9.10319 sec
literal by value: 100000000 iterations -> time elapsed: 8.62659 sec
Unless you actually need a copy it's still reasonable to take const &. For example:
bool isprint(std::string const &s) {
return all_of(begin(s),end(s),(bool(*)(char))isprint);
}
If you change this to take the string by value then you'll end up moving or copying the parameter, and there's no need for that. Not only is copy/move likely more expensive, but it also introduces a new potential failure; the copy/move could throw an exception (e.g., allocation during copy could fail) whereas taking a reference to an existing value can't.
If you do need a copy then passing and returning by value is usually (always?) the best option. In fact I generally wouldn't worry about it in C++03 unless you find that extra copies actually causes a performance problem. Copy elision seems pretty reliable on modern compilers. I think people's skepticism and insistence that you have to check your table of compiler support for RVO is mostly obsolete nowadays.
In short, C++11 doesn't really change anything in this regard except for people that didn't trust copy elision.
Almost.
In C++17, we have basic_string_view<?>, which brings us down to basically one narrow use case for std::string const& parameters.
The existence of move semantics has eliminated one use case for std::string const& -- if you are planning on storing the parameter, taking a std::string by value is more optimal, as you can move out of the parameter.
If someone called your function with a raw C "string" this means only one std::string buffer is ever allocated, as opposed to two in the std::string const& case.
However, if you don't intend to make a copy, taking by std::string const& is still useful in C++14.
With std::string_view, so long as you aren't passing said string to an API that expects C-style '\0'-terminated character buffers, you can more efficiently get std::string like functionality without risking any allocation. A raw C string can even be turned into a std::string_view without any allocation or character copying.
At that point, the use for std::string const& is when you aren't copying the data wholesale, and are going to pass it on to a C-style API that expects a null terminated buffer, and you need the higher level string functions that std::string provides. In practice, this is a rare set of requirements.
std::string is not Plain Old Data(POD), and its raw size is not the most relevant thing ever. For example, if you pass in a string which is above the length of SSO and allocated on the heap, I would expect the copy constructor to not copy the SSO storage.
The reason this is recommended is because inval is constructed from the argument expression, and thus is always moved or copied as appropriate- there is no performance loss, assuming that you need ownership of the argument. If you don't, a const reference could still be the better way to go.
I've copy/pasted the answer from this question here, and changed the names and spelling to fit this question.
Here is code to measure what is being asked:
#include <iostream>
struct string
{
string() {}
string(const string&) {std::cout << "string(const string&)\n";}
string& operator=(const string&) {std::cout << "string& operator=(const string&)\n";return *this;}
#if (__has_feature(cxx_rvalue_references))
string(string&&) {std::cout << "string(string&&)\n";}
string& operator=(string&&) {std::cout << "string& operator=(string&&)\n";return *this;}
#endif
};
#if PROCESS == 1
string
do_something(string inval)
{
// do stuff
return inval;
}
#elif PROCESS == 2
string
do_something(const string& inval)
{
string return_val = inval;
// do stuff
return return_val;
}
#if (__has_feature(cxx_rvalue_references))
string
do_something(string&& inval)
{
// do stuff
return std::move(inval);
}
#endif
#endif
string source() {return string();}
int main()
{
std::cout << "do_something with lvalue:\n\n";
string x;
string t = do_something(x);
#if (__has_feature(cxx_rvalue_references))
std::cout << "\ndo_something with xvalue:\n\n";
string u = do_something(std::move(x));
#endif
std::cout << "\ndo_something with prvalue:\n\n";
string v = do_something(source());
}
For me this outputs:
$ clang++ -std=c++11 -stdlib=libc++ -DPROCESS=1 test.cpp
$ a.out
do_something with lvalue:
string(const string&)
string(string&&)
do_something with xvalue:
string(string&&)
string(string&&)
do_something with prvalue:
string(string&&)
$ clang++ -std=c++11 -stdlib=libc++ -DPROCESS=2 test.cpp
$ a.out
do_something with lvalue:
string(const string&)
do_something with xvalue:
string(string&&)
do_something with prvalue:
string(string&&)
The table below summarizes my results (using clang -std=c++11). The first number is the number of copy constructions and the second number is the number of move constructions:
+----+--------+--------+---------+
| | lvalue | xvalue | prvalue |
+----+--------+--------+---------+
| p1 | 1/1 | 0/2 | 0/1 |
+----+--------+--------+---------+
| p2 | 1/0 | 0/1 | 0/1 |
+----+--------+--------+---------+
The pass-by-value solution requires only one overload but costs an extra move construction when passing lvalues and xvalues. This may or may not be acceptable for any given situation. Both solutions have advantages and disadvantages.
Herb Sutter is still on record, along with Bjarne Stroustroup, in recommending const std::string& as a parameter type; see https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rf-in .
There is a pitfall not mentioned in any of the other answers here: if you pass a string literal to a const std::string& parameter, it will pass a reference to a temporary string, created on-the-fly to hold the characters of the literal. If you then save that reference, it will be invalid once the temporary string is deallocated. To be safe, you must save a copy, not the reference. The problem stems from the fact that string literals are const char[N] types, requiring promotion to std::string.
The code below illustrates the pitfall and the workaround, along with a minor efficiency option -- overloading with a const char* method, as described at Is there a way to pass a string literal as reference in C++.
(Note: Sutter & Stroustroup advise that if you keep a copy of the string, also provide an overloaded function with a && parameter and std::move() it.)
#include <string>
#include <iostream>
class WidgetBadRef {
public:
WidgetBadRef(const std::string& s) : myStrRef(s) // copy the reference...
{}
const std::string& myStrRef; // might be a reference to a temporary (oops!)
};
class WidgetSafeCopy {
public:
WidgetSafeCopy(const std::string& s) : myStrCopy(s)
// constructor for string references; copy the string
{std::cout << "const std::string& constructor\n";}
WidgetSafeCopy(const char* cs) : myStrCopy(cs)
// constructor for string literals (and char arrays);
// for minor efficiency only;
// create the std::string directly from the chars
{std::cout << "const char * constructor\n";}
const std::string myStrCopy; // save a copy, not a reference!
};
int main() {
WidgetBadRef w1("First string");
WidgetSafeCopy w2("Second string"); // uses the const char* constructor, no temp string
WidgetSafeCopy w3(w2.myStrCopy); // uses the String reference constructor
std::cout << w1.myStrRef << "\n"; // garbage out
std::cout << w2.myStrCopy << "\n"; // OK
std::cout << w3.myStrCopy << "\n"; // OK
}
OUTPUT:
const char * constructor
const std::string& constructor
Second string
Second string
See “Herb Sutter "Back to the Basics! Essentials of Modern C++ Style”. Among other topics, he reviews the parameter passing advice that’s been given in the past, and new ideas that come in with C++11 and specifically looks at the idea of passing strings by value.
The benchmarks show that passing std::strings by value, in cases where the function will copy it in anyway, can be significantly slower!
This is because you are forcing it to always make a full copy (and then move into place), while the const& version will update the old string which may reuse the already-allocated buffer.
See his slide 27: For “set” functions, option 1 is the same as it always was. Option 2 adds an overload for rvalue reference, but this gives a combinatorial explosion if there are multiple parameters.
It is only for “sink” parameters where a string must be created (not have its existing value changed) that the pass-by-value trick is valid. That is, constructors in which the parameter directly initializes the member of the matching type.
If you want to see how deep you can go in worrying about this, watch Nicolai Josuttis’s presentation and good luck with that (“Perfect — Done!” n times after finding fault with the previous version. Ever been there?)
This is also summarized as ⧺F.15 in the Standard Guidelines.
update
Generally, you want to declare "string" parameters as std::string_view (by value). This allows you to pass an existing std::string object as efficiently as with const std::string&, and also pass a lexical string literal (like "hello!") without copying it, and pass objects of type string_view which is necessary now that those are in the ecosystem too.
The exception is when the function needs an actual std::string instance, in order to pass to another function that's declared to take const std::string&.
IMO using the C++ reference for std::string is a quick and short local optimization, while using passing by value could be (or not) a better global optimization.
So the answer is: it depends on circumstances:
If you write all the code from the outside to the inside functions, you know what the code does, you can use the reference const std::string &.
If you write the library code or use heavily library code where strings are passed, you likely gain more in global sense by trusting std::string copy constructor behavior.
As #JDługosz points out in the comments, Herb gives other advice in another (later?) talk, see roughly from here: https://youtu.be/xnqTKD8uD64?t=54m50s.
His advice boils down to only using value parameters for a function f that takes so-called sink arguments, assuming you will move construct from these sink arguments.
This general approach only adds the overhead of a move constructor for both lvalue and rvalue arguments compared to an optimal implementation of f tailored to lvalue and rvalue arguments respectively. To see why this is the case, suppose f takes a value parameter, where T is some copy and move constructible type:
void f(T x) {
T y{std::move(x)};
}
Calling f with an lvalue argument will result in a copy constructor being called to construct x, and a move constructor being called to construct y. On the other hand, calling f with an rvalue argument will cause a move constructor to be called to construct x, and another move constructor to be called to construct y.
In general, the optimal implementation of f for lvalue arguments is as follows:
void f(const T& x) {
T y{x};
}
In this case, only one copy constructor is called to construct y. The optimal implementation of f for rvalue arguments is, again in general, as follows:
void f(T&& x) {
T y{std::move(x)};
}
In this case, only one move constructor is called to construct y.
So a sensible compromise is to take a value parameter and have one extra move constructor call for either lvalue or rvalue arguments with respect to the optimal implementation, which is also the advice given in Herb's talk.
As #JDługosz pointed out in the comments, passing by value only makes sense for functions that will construct some object from the sink argument. When you have a function f that copies its argument, the pass-by-value approach will have more overhead than a general pass-by-const-reference approach. The pass-by-value approach for a function f that retains a copy of its parameter will have the form:
void f(T x) {
T y{...};
...
y = std::move(x);
}
In this case, there is a copy construction and a move assignment for an lvalue argument, and a move construction and move assignment for an rvalue argument. The most optimal case for an lvalue argument is:
void f(const T& x) {
T y{...};
...
y = x;
}
This boils down to an assignment only, which is potentially much cheaper than the copy constructor plus move assignment required for the pass-by-value approach. The reason for this is that the assignment might reuse existing allocated memory in y, and therefore prevent (de)allocations, whereas the copy constructor will usually allocate memory.
For an rvalue argument the most optimal implementation for f that retains a copy has the form:
void f(T&& x) {
T y{...};
...
y = std::move(x);
}
So, only a move assignment in this case. Passing an rvalue to the version of f that takes a const reference only costs an assignment instead of a move assignment. So relatively speaking, the version of f taking a const reference in this case as the general implementation is preferable.
So in general, for the most optimal implementation, you will need to overload or do some kind of perfect forwarding as shown in the talk. The drawback is a combinatorial explosion in the number of overloads required, depending on the number of parameters for f in case you opt to overload on the value category of the argument. Perfect forwarding has the drawback that f becomes a template function, which prevents making it virtual, and results in significantly more complex code if you want to get it 100% right (see the talk for the gory details).
The problem is that "const" is a non-granular qualifier. What is usually meant by "const string ref" is "don't modify this string", not "don't modify the reference count". There is simply no way, in C++, to say which members are "const". They either all are, or none of them are.
In order to hack around this language issue, STL could allow "C()" in your example to make a move-semantic copy anyway, and dutifully ignore the "const" with regard to the reference count (mutable). As long as it was well-specified, this would be fine.
Since STL doesn't, I have a version of a string that const_casts<> away the reference counter (no way to retroactively make something mutable in a class hierarchy), and - lo and behold - you can freely pass cmstring's as const references, and make copies of them in deep functions, all day long, with no leaks or issues.
Since C++ offers no "derived class const granularity" here, writing up a good specification and making a shiny new "const movable string" (cmstring) object is the best solution I've seen.
I'm telling a longwinded backstory here because, in addition to a direct answer, I'd like to know if my reasoning which led into this situation was correct.
I have a function taking a dynamic_bitset<> argument (from Boost.dynamic_bitset).
Say it looks like this.
void foo(boost::dynamic_bitset<> db) {
// do stuff
}
It so happens that it's only ever called with temporaries, built from the constructor, as in foo(boost::dynamic_bitset<>{5}.set()) (to call with a 5-bit bitset with all bits set).
My bitsets only have a small number of bits (fewer than 32). So at first, I thought "I'll just pass it by value; the copy is smaller than a pointer." But then I thought "It's dynamic, so it must allocate space on the heap. I want to avoid unnecessary allocations and frees."
So, I could make it
void foo(const boost::dynamic_bitset<>& db);
But a reference is a pointer, and a dynamic_bitset (presumably) has a pointer to its data, so then using db within foo would go through two levels of indirection, which seems dumb. Clearly the best way would be to copy the pointer to the data into foo, without reallocating and copying the data on the heap.
"Aha!" I say. "Surely this is what move semantics are for." So, I change the signature to
void foo(boost::dynamic_bitset<>&& db);
But then, calling foo(boost::dynamic_bitset<>{5}.set()) gives a compiler error, cannot bind 'boost::dynamic_bitset<>' lvalue to 'boost::dynamic_bitset<>&&'.
I must instead call
foo(std::move(boost::dynamic_bitset<>{5}.set()))
and then everything works.
Why do I need to call std::move?
This seems like it's clearly an xvalue (a temporary about to expire), doesn't it?
From the boost docs:
dynamic_bitset& set();
set() returns an lvalue.
In general, if a function returns by value, or by rvalue reference:
dynamic_bitset set();
dynamic_bitset&& set();
then the expression set() is an rvalue (in the first case a prvalue, and in the second an xvalue). However because in the actual code set() returns an lvalue reference, the expression set() is an lvalue.
category values http://downloads.sehe.nl/stackoverflow/value-categories.svg
I heard a recent talk by Herb Sutter who suggested that the reasons to pass std::vector and std::string by const & are largely gone. He suggested that writing a function such as the following is now preferable:
std::string do_something ( std::string inval )
{
std::string return_val;
// ... do stuff ...
return return_val;
}
I understand that the return_val will be an rvalue at the point the function returns and can therefore be returned using move semantics, which are very cheap. However, inval is still much larger than the size of a reference (which is usually implemented as a pointer). This is because a std::string has various components including a pointer into the heap and a member char[] for short string optimization. So it seems to me that passing by reference is still a good idea.
Can anyone explain why Herb might have said this?
The reason Herb said what he said is because of cases like this.
Let's say I have function A which calls function B, which calls function C. And A passes a string through B and into C. A does not know or care about C; all A knows about is B. That is, C is an implementation detail of B.
Let's say that A is defined as follows:
void A()
{
B("value");
}
If B and C take the string by const&, then it looks something like this:
void B(const std::string &str)
{
C(str);
}
void C(const std::string &str)
{
//Do something with `str`. Does not store it.
}
All well and good. You're just passing pointers around, no copying, no moving, everyone's happy. C takes a const& because it doesn't store the string. It simply uses it.
Now, I want to make one simple change: C needs to store the string somewhere.
void C(const std::string &str)
{
//Do something with `str`.
m_str = str;
}
Hello, copy constructor and potential memory allocation (ignore the Short String Optimization (SSO)). C++11's move semantics are supposed to make it possible to remove needless copy-constructing, right? And A passes a temporary; there's no reason why C should have to copy the data. It should just abscond with what was given to it.
Except it can't. Because it takes a const&.
If I change C to take its parameter by value, that just causes B to do the copy into that parameter; I gain nothing.
So if I had just passed str by value through all of the functions, relying on std::move to shuffle the data around, we wouldn't have this problem. If someone wants to hold on to it, they can. If they don't, oh well.
Is it more expensive? Yes; moving into a value is more expensive than using references. Is it less expensive than the copy? Not for small strings with SSO. Is it worth doing?
It depends on your use case. How much do you hate memory allocations?
Are the days of passing const std::string & as a parameter over?
No. Many people take this advice (including Dave Abrahams) beyond the domain it applies to, and simplify it to apply to all std::string parameters -- Always passing std::string by value is not a "best practice" for any and all arbitrary parameters and applications because the optimizations these talks/articles focus on apply only to a restricted set of cases.
If you're returning a value, mutating the parameter, or taking the value, then passing by value could save expensive copying and offer syntactical convenience.
As ever, passing by const reference saves much copying when you don't need a copy.
Now to the specific example:
However inval is still quite a lot larger than the size of a reference (which is usually implemented as a pointer). This is because a std::string has various components including a pointer into the heap and a member char[] for short string optimization. So it seems to me that passing by reference is still a good idea. Can anyone explain why Herb might have said this?
If stack size is a concern (and assuming this is not inlined/optimized), return_val + inval > return_val -- IOW, peak stack usage can be reduced by passing by value here (note: oversimplification of ABIs). Meanwhile, passing by const reference can disable the optimizations. The primary reason here is not to avoid stack growth, but to ensure the optimization can be performed where it is applicable.
The days of passing by const reference aren't over -- the rules just more complicated than they once were. If performance is important, you'll be wise to consider how you pass these types, based on the details you use in your implementations.
Short answer: NO! Long answer:
If you won't modify the string (treat is as read-only), pass it as const ref&.(the const ref& obviously needs to stay within scope while the function that uses it executes)
If you plan to modify it or you know it will get out of scope (threads), pass it as a value, don't copy the const ref& inside your function body.
There was a post on cpp-next.com called "Want speed, pass by value!". The TL;DR:
Guideline: Don’t copy your function arguments. Instead, pass them by value and let the compiler do the copying.
TRANSLATION of ^
Don’t copy your function arguments --- means: if you plan to modify the argument value by copying it to an internal variable, just use a value argument instead.
So, don't do this:
std::string function(const std::string& aString){
auto vString(aString);
vString.clear();
return vString;
}
do this:
std::string function(std::string aString){
aString.clear();
return aString;
}
When you need to modify the argument value in your function body.
You just need to be aware how you plan to use the argument in the function body. Read-only or NOT... and if it sticks within scope.
This highly depends on the compiler's implementation.
However, it also depends on what you use.
Lets consider next functions :
bool foo1( const std::string v )
{
return v.empty();
}
bool foo2( const std::string & v )
{
return v.empty();
}
These functions are implemented in a separate compilation unit in order to avoid inlining. Then :
1. If you pass a literal to these two functions, you will not see much difference in performances. In both cases, a string object has to be created
2. If you pass another std::string object, foo2 will outperform foo1, because foo1 will do a deep copy.
On my PC, using g++ 4.6.1, I got these results :
variable by reference: 1000000000 iterations -> time elapsed: 2.25912 sec
variable by value: 1000000000 iterations -> time elapsed: 27.2259 sec
literal by reference: 100000000 iterations -> time elapsed: 9.10319 sec
literal by value: 100000000 iterations -> time elapsed: 8.62659 sec
Unless you actually need a copy it's still reasonable to take const &. For example:
bool isprint(std::string const &s) {
return all_of(begin(s),end(s),(bool(*)(char))isprint);
}
If you change this to take the string by value then you'll end up moving or copying the parameter, and there's no need for that. Not only is copy/move likely more expensive, but it also introduces a new potential failure; the copy/move could throw an exception (e.g., allocation during copy could fail) whereas taking a reference to an existing value can't.
If you do need a copy then passing and returning by value is usually (always?) the best option. In fact I generally wouldn't worry about it in C++03 unless you find that extra copies actually causes a performance problem. Copy elision seems pretty reliable on modern compilers. I think people's skepticism and insistence that you have to check your table of compiler support for RVO is mostly obsolete nowadays.
In short, C++11 doesn't really change anything in this regard except for people that didn't trust copy elision.
Almost.
In C++17, we have basic_string_view<?>, which brings us down to basically one narrow use case for std::string const& parameters.
The existence of move semantics has eliminated one use case for std::string const& -- if you are planning on storing the parameter, taking a std::string by value is more optimal, as you can move out of the parameter.
If someone called your function with a raw C "string" this means only one std::string buffer is ever allocated, as opposed to two in the std::string const& case.
However, if you don't intend to make a copy, taking by std::string const& is still useful in C++14.
With std::string_view, so long as you aren't passing said string to an API that expects C-style '\0'-terminated character buffers, you can more efficiently get std::string like functionality without risking any allocation. A raw C string can even be turned into a std::string_view without any allocation or character copying.
At that point, the use for std::string const& is when you aren't copying the data wholesale, and are going to pass it on to a C-style API that expects a null terminated buffer, and you need the higher level string functions that std::string provides. In practice, this is a rare set of requirements.
std::string is not Plain Old Data(POD), and its raw size is not the most relevant thing ever. For example, if you pass in a string which is above the length of SSO and allocated on the heap, I would expect the copy constructor to not copy the SSO storage.
The reason this is recommended is because inval is constructed from the argument expression, and thus is always moved or copied as appropriate- there is no performance loss, assuming that you need ownership of the argument. If you don't, a const reference could still be the better way to go.
I've copy/pasted the answer from this question here, and changed the names and spelling to fit this question.
Here is code to measure what is being asked:
#include <iostream>
struct string
{
string() {}
string(const string&) {std::cout << "string(const string&)\n";}
string& operator=(const string&) {std::cout << "string& operator=(const string&)\n";return *this;}
#if (__has_feature(cxx_rvalue_references))
string(string&&) {std::cout << "string(string&&)\n";}
string& operator=(string&&) {std::cout << "string& operator=(string&&)\n";return *this;}
#endif
};
#if PROCESS == 1
string
do_something(string inval)
{
// do stuff
return inval;
}
#elif PROCESS == 2
string
do_something(const string& inval)
{
string return_val = inval;
// do stuff
return return_val;
}
#if (__has_feature(cxx_rvalue_references))
string
do_something(string&& inval)
{
// do stuff
return std::move(inval);
}
#endif
#endif
string source() {return string();}
int main()
{
std::cout << "do_something with lvalue:\n\n";
string x;
string t = do_something(x);
#if (__has_feature(cxx_rvalue_references))
std::cout << "\ndo_something with xvalue:\n\n";
string u = do_something(std::move(x));
#endif
std::cout << "\ndo_something with prvalue:\n\n";
string v = do_something(source());
}
For me this outputs:
$ clang++ -std=c++11 -stdlib=libc++ -DPROCESS=1 test.cpp
$ a.out
do_something with lvalue:
string(const string&)
string(string&&)
do_something with xvalue:
string(string&&)
string(string&&)
do_something with prvalue:
string(string&&)
$ clang++ -std=c++11 -stdlib=libc++ -DPROCESS=2 test.cpp
$ a.out
do_something with lvalue:
string(const string&)
do_something with xvalue:
string(string&&)
do_something with prvalue:
string(string&&)
The table below summarizes my results (using clang -std=c++11). The first number is the number of copy constructions and the second number is the number of move constructions:
+----+--------+--------+---------+
| | lvalue | xvalue | prvalue |
+----+--------+--------+---------+
| p1 | 1/1 | 0/2 | 0/1 |
+----+--------+--------+---------+
| p2 | 1/0 | 0/1 | 0/1 |
+----+--------+--------+---------+
The pass-by-value solution requires only one overload but costs an extra move construction when passing lvalues and xvalues. This may or may not be acceptable for any given situation. Both solutions have advantages and disadvantages.
Herb Sutter is still on record, along with Bjarne Stroustroup, in recommending const std::string& as a parameter type; see https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rf-in .
There is a pitfall not mentioned in any of the other answers here: if you pass a string literal to a const std::string& parameter, it will pass a reference to a temporary string, created on-the-fly to hold the characters of the literal. If you then save that reference, it will be invalid once the temporary string is deallocated. To be safe, you must save a copy, not the reference. The problem stems from the fact that string literals are const char[N] types, requiring promotion to std::string.
The code below illustrates the pitfall and the workaround, along with a minor efficiency option -- overloading with a const char* method, as described at Is there a way to pass a string literal as reference in C++.
(Note: Sutter & Stroustroup advise that if you keep a copy of the string, also provide an overloaded function with a && parameter and std::move() it.)
#include <string>
#include <iostream>
class WidgetBadRef {
public:
WidgetBadRef(const std::string& s) : myStrRef(s) // copy the reference...
{}
const std::string& myStrRef; // might be a reference to a temporary (oops!)
};
class WidgetSafeCopy {
public:
WidgetSafeCopy(const std::string& s) : myStrCopy(s)
// constructor for string references; copy the string
{std::cout << "const std::string& constructor\n";}
WidgetSafeCopy(const char* cs) : myStrCopy(cs)
// constructor for string literals (and char arrays);
// for minor efficiency only;
// create the std::string directly from the chars
{std::cout << "const char * constructor\n";}
const std::string myStrCopy; // save a copy, not a reference!
};
int main() {
WidgetBadRef w1("First string");
WidgetSafeCopy w2("Second string"); // uses the const char* constructor, no temp string
WidgetSafeCopy w3(w2.myStrCopy); // uses the String reference constructor
std::cout << w1.myStrRef << "\n"; // garbage out
std::cout << w2.myStrCopy << "\n"; // OK
std::cout << w3.myStrCopy << "\n"; // OK
}
OUTPUT:
const char * constructor
const std::string& constructor
Second string
Second string
See “Herb Sutter "Back to the Basics! Essentials of Modern C++ Style”. Among other topics, he reviews the parameter passing advice that’s been given in the past, and new ideas that come in with C++11 and specifically looks at the idea of passing strings by value.
The benchmarks show that passing std::strings by value, in cases where the function will copy it in anyway, can be significantly slower!
This is because you are forcing it to always make a full copy (and then move into place), while the const& version will update the old string which may reuse the already-allocated buffer.
See his slide 27: For “set” functions, option 1 is the same as it always was. Option 2 adds an overload for rvalue reference, but this gives a combinatorial explosion if there are multiple parameters.
It is only for “sink” parameters where a string must be created (not have its existing value changed) that the pass-by-value trick is valid. That is, constructors in which the parameter directly initializes the member of the matching type.
If you want to see how deep you can go in worrying about this, watch Nicolai Josuttis’s presentation and good luck with that (“Perfect — Done!” n times after finding fault with the previous version. Ever been there?)
This is also summarized as ⧺F.15 in the Standard Guidelines.
update
Generally, you want to declare "string" parameters as std::string_view (by value). This allows you to pass an existing std::string object as efficiently as with const std::string&, and also pass a lexical string literal (like "hello!") without copying it, and pass objects of type string_view which is necessary now that those are in the ecosystem too.
The exception is when the function needs an actual std::string instance, in order to pass to another function that's declared to take const std::string&.
IMO using the C++ reference for std::string is a quick and short local optimization, while using passing by value could be (or not) a better global optimization.
So the answer is: it depends on circumstances:
If you write all the code from the outside to the inside functions, you know what the code does, you can use the reference const std::string &.
If you write the library code or use heavily library code where strings are passed, you likely gain more in global sense by trusting std::string copy constructor behavior.
As #JDługosz points out in the comments, Herb gives other advice in another (later?) talk, see roughly from here: https://youtu.be/xnqTKD8uD64?t=54m50s.
His advice boils down to only using value parameters for a function f that takes so-called sink arguments, assuming you will move construct from these sink arguments.
This general approach only adds the overhead of a move constructor for both lvalue and rvalue arguments compared to an optimal implementation of f tailored to lvalue and rvalue arguments respectively. To see why this is the case, suppose f takes a value parameter, where T is some copy and move constructible type:
void f(T x) {
T y{std::move(x)};
}
Calling f with an lvalue argument will result in a copy constructor being called to construct x, and a move constructor being called to construct y. On the other hand, calling f with an rvalue argument will cause a move constructor to be called to construct x, and another move constructor to be called to construct y.
In general, the optimal implementation of f for lvalue arguments is as follows:
void f(const T& x) {
T y{x};
}
In this case, only one copy constructor is called to construct y. The optimal implementation of f for rvalue arguments is, again in general, as follows:
void f(T&& x) {
T y{std::move(x)};
}
In this case, only one move constructor is called to construct y.
So a sensible compromise is to take a value parameter and have one extra move constructor call for either lvalue or rvalue arguments with respect to the optimal implementation, which is also the advice given in Herb's talk.
As #JDługosz pointed out in the comments, passing by value only makes sense for functions that will construct some object from the sink argument. When you have a function f that copies its argument, the pass-by-value approach will have more overhead than a general pass-by-const-reference approach. The pass-by-value approach for a function f that retains a copy of its parameter will have the form:
void f(T x) {
T y{...};
...
y = std::move(x);
}
In this case, there is a copy construction and a move assignment for an lvalue argument, and a move construction and move assignment for an rvalue argument. The most optimal case for an lvalue argument is:
void f(const T& x) {
T y{...};
...
y = x;
}
This boils down to an assignment only, which is potentially much cheaper than the copy constructor plus move assignment required for the pass-by-value approach. The reason for this is that the assignment might reuse existing allocated memory in y, and therefore prevent (de)allocations, whereas the copy constructor will usually allocate memory.
For an rvalue argument the most optimal implementation for f that retains a copy has the form:
void f(T&& x) {
T y{...};
...
y = std::move(x);
}
So, only a move assignment in this case. Passing an rvalue to the version of f that takes a const reference only costs an assignment instead of a move assignment. So relatively speaking, the version of f taking a const reference in this case as the general implementation is preferable.
So in general, for the most optimal implementation, you will need to overload or do some kind of perfect forwarding as shown in the talk. The drawback is a combinatorial explosion in the number of overloads required, depending on the number of parameters for f in case you opt to overload on the value category of the argument. Perfect forwarding has the drawback that f becomes a template function, which prevents making it virtual, and results in significantly more complex code if you want to get it 100% right (see the talk for the gory details).
The problem is that "const" is a non-granular qualifier. What is usually meant by "const string ref" is "don't modify this string", not "don't modify the reference count". There is simply no way, in C++, to say which members are "const". They either all are, or none of them are.
In order to hack around this language issue, STL could allow "C()" in your example to make a move-semantic copy anyway, and dutifully ignore the "const" with regard to the reference count (mutable). As long as it was well-specified, this would be fine.
Since STL doesn't, I have a version of a string that const_casts<> away the reference counter (no way to retroactively make something mutable in a class hierarchy), and - lo and behold - you can freely pass cmstring's as const references, and make copies of them in deep functions, all day long, with no leaks or issues.
Since C++ offers no "derived class const granularity" here, writing up a good specification and making a shiny new "const movable string" (cmstring) object is the best solution I've seen.
When writing a C++ function which has args that are being passed to it, from my understanding const should always be used if you can guarantuee that the object will not be changed or a const pointer if the pointer won't be changed.
When else is this practice advised?
When would you use a const reference and what are the advantages over just passing it through a pointer for example?
What about this void MyObject::Somefunc(const std::string& mystring) What would be the point in having a const string if a string is in fact already an immutable object?
Asking whether to add const is the wrong question, unfortunately.
Compare non-const ref to passing a non-const pointer
void modifies(T ¶m);
void modifies(T *param);
This case is mostly about style: do you want the call to look like call(obj) or call(&obj)? However, there are two points where the difference matters. If you want to be able to pass null, you must use a pointer. And if you're overloading operators, you cannot use a pointer instead.
Compare const ref to by value
void doesnt_modify(T const ¶m);
void doesnt_modify(T param);
This is the interesting case. The rule of thumb is "cheap to copy" types are passed by value — these are generally small types (but not always) — while others are passed by const ref. However, if you need to make a copy within your function regardless, you should pass by value. (Yes, this exposes a bit of implementation detail. C'est le C++.)
Compare const pointer to non-modifying plus overload
void optional(T const *param=0);
// vs
void optional();
void optional(T const ¶m); // or optional(T param)
This is related to the non-modifying case above, except passing the parameter is optional. There's the least difference here between all three situations, so choose whichever makes your life easiest. Of course, the default value for the non-const pointer is up to you.
Const by value is an implementation detail
void f(T);
void f(T const);
These declarations are actually the exact same function! When passing by value, const is purely an implementation detail. Try it out:
void f(int);
void f(int const) {/*implements above function, not an overload*/}
typedef void C(int const);
typedef void NC(int);
NC *nc = &f; // nc is a function pointer
C *c = nc; // C and NC are identical types
The general rule is, use const whenever possible, and only omit it if necessary. const may enable the compiler to optimize and helps your peers understand how your code is intended to be used (and the compiler will catch possible misuse).
As for your example, strings are not immutable in C++. If you hand a non-const reference to a string to a function, the function may modify it. C++ does not have the concept of immutability built into the language, you can only emulate it using encapsulation and const (which will never be bullet-proof though).
After thinking #Eamons comment and reading some stuff, I agree that optimization is not the main reason for using const. The main reason is to have correct code.
The questions are based on some incorrect assumptions, so not really meaningful.
std::string does not model immutable string values. It models mutable values.
There is no such thing as a "const reference". There are references to const objects. The distinction is subtle but important.
Top-level const for a function argument is only meaningful for a function implementation, not for a pure declaration (where it's disregarded by the compiler). It doesn't tell the caller anything. It's only a restriction on the implementation. E.g. int const is pretty much meaningless as argument type in a pure declaration of a function. However, the const in std::string const& is not top level.
Passing by reference to const avoids inefficient copying of data. In general, for an argument passing data into a function, you pass small items (such as an int) by value, and potentially larger items by reference to const. In the machine code the reference to const may be optimized away or it may be implemented as a pointer. E.g., in 32-bit Windows an int is 4 bytes and a pointer is 4 bytes. So argument type int const& would not reduce data copying but could, with a simple-minded compiler, introduce an extra indirection, which means a slight inefficiency -- hence the small/large distinction.
Cheers & hth.,
The main advantage of const reference over const pointer is following: its clear that the parameter is required and cannot be NULL.
Vice versa, if i see a const pointer, i immedeately assume the reason for it not being a reference is that the parameter could be NULL.