What is most appropriate in const std::string assignment/declaration? Using the constructor(e.g., const std::string WORD("hello");) or using equal operator(e.g., const std::string WORD= "hello";)?
Does these things has difference in memory usage or time processes?
For any reasonable compiler, the code generated will be the same in both cases. Whether you should use direct initialization or copy-initialization in this case is essentially opinion-based.
In both cases, generally compilers will remove the copies using "Return Value Optimisation" anyway. See this code on ideone here calls neither the ordinary constructor nor the assignment operator as it doesn't print that they're being called to screen:
That is:
#include <iostream>
class C
{
public:
C() {}
C(const char[]) { std::cout << "Ordinary Constructor" << std::endl; }
C& operator=(const char[]) { std::cout << "Assignment Operator" << std::endl; return *this; }
};
int main() {
std::cout << "Start" << std::endl;
C x1 = "Hello";
C x2("Hello");
std::cout << "End" << std::endl;
}
Simply outputs:
Start
End
It doesn't output:
Start
Assignment Operator
Ordinary Constructor
End
As C++ allows the copies to be skipped and the temporary to be constructed in place.
The lines:
std::string x = "hello";
std::string x("hello");
both will only call the constructor of std::string. That is, they are identical, and neither will use the operator= overloads.
Related
This question is already asked most likely, but I did not find the answer.
The code below compiles with gcc but crashes at runtime, with std::length_error (live).
void test(const std::string &value) { std::cout << "string overload: " << value << std::endl; }
//void test(const std::vector<std::string> &) { std::cout << "vector overload" << std::endl; }
int main()
{
test({"one", "two"});
}
The ability to create a string from the initializer list of strings seems controversial and, for example, does not make it possible to create the overload commented out in the code above.
But even if such construction is allowed, why does it lead to a failure?
It calls
string(const char* b, const char* e)
string ctor overload.
It works only if b and e points to the same string literal. Otherwise it is undefined behaviour.
For starters there is no used the constructor that accepts an initializer list because such a constructor looks like
basic_string(initializer_list<charT>, const Allocator& = Allocator());
^^^^^
So the compiler searches another appropriate constructor and it finds such a constructor. It is the constructor
template<class InputIterator>
basic_string(InputIterator begin, InputIterator end, const Allocator& a = Allocator());
That is the expressions "one" and "two" are considered as iterators of the type const char *.
So the function test has undefined behavior.
You could write for example (provided that string literals with the same content are stored as one string literal in memory, which is not guaranteed and depends on the selected compiler options).
#include <iostream>
#include <string>
void test(const std::string &value) { std::cout << "string overload: " << value << std::endl; }
//void test(const std::vector<std::string> &) { std::cout << "vector overload" << std::endl; }
int main()
{
test({ "one", "one" + 3 });
}
And you will get a valid result.
string overload: one
Pay attention to that this construction
{ "one", "two" }
is not an object of the type std::initializer_list<T>. This construction does not have a type. It is a braced-init-list that is used as an initialzer. Simply the compiler tries at first to use a constructor that have the first parameter of the type std::initializer_list to use with this initializer.
For example if you will use the class std::vector<const char *> then indeed the compiler will use its constructor with std::initializer_list and correspondingly initializes its parameter with this braced-init-list. For example
#include <iostream>
#include <vector>
int main()
{
std::vector<const char *> v( { "one", "two" } );
for ( const auto &s : v ) std::cout << s << ' ';
std::cout << '\n';
}
Can someone explain why the original object that is passed to a new object via std::move is still valid afterwards?
#include <iostream>
class Class
{
public:
explicit Class(const double& tt) : m_type(tt)
{
std::cout << "defaultish" << std::endl;
};
explicit Class(const Class& val) :
m_type(val.m_type)
{
std::cout << "copy" << std::endl;
};
explicit Class(Class&& val) :
m_type(val.m_type)
{
m_type = val.m_type;
std::cout << "move: " << m_type << std::endl;
};
void print()
{
std::cout << "print: " << m_type << std::endl;
}
void set(const double& tt)
{
m_type = tt;
}
private:
double m_type;
};
int main ()
{
Class cc(3.2);
Class c2(std::move(cc));
c2.print();
cc.set(4.0);
cc.print();
return 0;
}
It outputs the following:
defaultish
move: 3.2
print: 3.2
print: 4
I would expect the calls to cc.set() and cc.print() to fail...
UPDATE
Thanks to answers below, we've identified that 1) I wasn't moving anything in the move constructor, and 2) std::move() on an int or double doesn't do anything because it's more expensive to move these types than to simply copy. The new code below updates the class's private member variable to be of type std::string instead of a double, and properly calls std::move when setting this private member variable in the Class' move constructor, resulting in an output that shows how a std::move results in a valid but unspecified state
#include <iostream>
#include <string>
class Class
{
public:
explicit Class(const std::string& tt) : m_type(tt)
{
std::cout << "defaultish" << std::endl;
};
explicit Class(const Class& val) :
m_type(val.m_type)
{
std::cout << "copy" << std::endl;
};
explicit Class(Class&& val) : m_type(std::move(val.m_type))
{
std::cout << "move: " << m_type << std::endl;
};
void print()
{
std::cout << "print: " << m_type << std::endl;
}
void set(const std::string val )
{
m_type = val;
}
private:
std::string m_type;
};
int main ()
{
Class cc("3.2");
Class c2(std::move(cc));
c2.print( );
cc.print();
cc.set( "4.0" );
cc.print();
return 0;
}
And finally the output:
defaultish
move: 3.2
print: 3.2
print:
print: 4.0
Because the standard says so.
Moved-from objects have a valid but unspecified state. That means you can still use them, but you can't be sure what state they'll be in. They could look just as they did before the move, depending on what is the most efficient way to "move" data out of them. For example, "moving" from an int makes no sense (you'd have to do extra work to reset the original value!) so a "move" from an int is actually only ever going to be a copy. The same is true of a double.
Although in this case it's got more to do with the fact that you didn't actually move anything.
In the code example, std::move determines which constructor gets called. Nothing more. So c2(std::move(cc)) calls the move constructor for Class. The move constructor for Class doesn't do anything to its argument, so cc is unchanged, and it can be used just as it could have before the call to the move constructor.
All the talk in comments and answers about the state of an object that has been moved from is about the requirements on standard library types, which will be left in a "valid but unspecified state" (17.6.5.15, [lib.types.movedfrom]). What you do with your types is not affected by that.
EDIT: sigh. You edited the code and changed the question. Now that your class holds a std::string instead of a float things are different, and the std::string object in cc is, indeed, in a "valid but unspecified state".
Right when I thought I understood what std::move and move constructors do, I tried to write some unit test, actually testing the move constructor for some class...
To my surprise I found, that I cannot think of a way to construct code which actually calls the move constructor. Worse, I cannot even set a breakpoint in the body of the move constructor (in VS2013 community edition, debug, 64bit build).
Wondering if this is a compiler peculiarity, I knocked up some small test code on my freebsd virtual machine, using clang (3.4.1). There, too I fail to find a way to get that move constructor invoked.
#include <iostream>
#include <stdint.h>
#include <string>
#include <algorithm>
#include <functional>
#include <ctype.h>
#include <locale>
void InPlaceToUpper( std::string& target )
{
std::transform(target.begin(), target.end(), target.begin(), ::toupper);
}
void InPlaceToLower( std::string& target )
{
std::transform(target.begin(), target.end(), target.begin(), ::tolower);
}
std::string ToUpper( const std::string& s )
{
std::string result;
result.resize(s.length());
std::transform(s.begin(), s.end(), result.begin(), ::toupper);
return result;
}
std::string ToLower( const std::string& s)
{
std::string result;
result.resize(s.length());
std::transform(s.begin(), s.end(), result.begin(), ::tolower);
return result;
}
class CFoo
{
std::string m_value;
public:
CFoo()
: m_value()
{
std::cout << "CFoo() called." << std::endl;
}
CFoo(const char *value)
: m_value(value)
{
std::cout << "CFoo(const char *) called." << std::endl;
}
CFoo(const std::string& value )
: m_value(value)
{
std::cout << "CFoo(const std::string&) called." << std::endl;
}
CFoo(const CFoo& other )
: m_value(other.m_value)
{
std::cout << "CFoo() copy constructor called." << std::endl;
}
CFoo(CFoo&& other )
: m_value(std::move(other.m_value))
{
std::cout << "CFoo() move constructor called." << std::endl;
std::cout << "other.m_value = " << other.m_value.c_str() << std::endl;
}
~CFoo()
{
std::cout << "~CFoo() called." << std::endl;
}
const CFoo& operator=( const CFoo& other )
{
std::cout << "CFoo copy assignment operator called." << std::endl;
if( &other != this )
{
m_value = other.m_value;
}
return *this;
}
const CFoo& operator=( CFoo&& other )
{
std::cout << "CFoo move assignment operator called." << std::endl;
if( &other != this )
{
m_value = std::move(other.m_value);
}
return *this;
}
CFoo ToUpper()
{
return CFoo(::ToUpper(m_value));
}
CFoo ToLower()
{
return CFoo(::ToLower(m_value));
}
const char * ToString() const
{
return m_value.c_str();
}
};
int main( int argc, const char *argv[] )
{
{
CFoo foo;
CFoo foo1("Hello World");
CFoo foo2 = CFoo("Hello again World!");
CFoo foo3(CFoo("Bye world"));
CFoo foo4 = CFoo("Bye again world");
CFoo foo5 = foo4.ToUpper();
CFoo foo6 = foo4.ToLower();
foo6 = foo4.ToUpper();
std::cout << "foo4: " << foo4.ToString() << std::endl;
foo6 = CFoo("Well well well");
}
return 0;
}
My apologies if the code is not as short as it might possibly be. But there are only a few spots to look at, namely my efforts to get the move constructor invoked in main() and the definition of the various constructors in class Foo.
I am aware of compiler settings which allow turning off RVO and stuff but for the purpose of using the feature "move constructor" in performance aware code, there should be a classic example of when it gets invoked. If that is not the case, I will probably decide not to even bother using move constructors at all.
To answer the question, you can tell me a line I can write in main() which gets the move constructor of CFoo called. Or you can tell me what I am doing wrong.
Does std::string support being moved like that? Maybe this is why my efforts fail?
Thanks, in advance.
In all your attempts to use the move constructor it is being elided, so that e.g. CFoo foo = CFoo(blah); is simply equivalent to CFoo foo(blah); which doesn't need to use the move constructor. This is a Good Thing because the compiler is optimising away the need for any copy or move to happen at all.
To see the move constructor being used try:
CFoo f1;
CFoo f2 = std::move(f1);
This constructs f2 from an rvalue, and nothing can be elided, so the move constructor will be used.
First of all, there is an error in std::string ToUpper(const std::string& s), namely there is no space in result. C++ algorithms do not grow their target containers automatically. You must either allocate space yourself, or use a inserter adaptor.
To make space in out, e.g. do this:
result.resize(s.length());
After it the move assignment operator called for:
foo6 = foo4.ToUpper();
foo6 = CFoo("Well well well");
The move constructor is called whenever an object is initialized from xvalue of the same type, which includes:
initialization, T a = std::move(b); or T a(std::move(b));, where b is of type T
function argument passing: f(std::move(a));, where a is of type T and f is void f(T t)
function return: return a; inside a function such as T f(), where a is of type T which has a move constructor.
For more see move constructors on cppreference.com.
I run this code for experimenting copy constructor and assignment operator
class AClass {
private:
int a;
public:
AClass (int a_) : a(a_) {
cout << " constructor AClass(int) " << a << endl;
}
AClass(const AClass & x) : a(x.a) {
cout << " copy constructor AClass(const AClass &) " << a << endl;
}
AClass & operator=(const AClass & x) {
a = x.a;
cout << " AClass& operator=(const AClass &) " << a - endl;
return *this;
}
};
AClass g () {
AClass x(8);
return x;
}
int main () {
cout << " before AClass b = g() " << endl;
AClass b = g();
cout << " after" << endl;
cout << " before AClass c(g()) " << endl;
AClass c (g());
cout << " after" << endl;
}
and found that no message appears for the return x;
Why?
Should not the copy constructor or operator= be called?
This is the output:
before AClass b = g()
constructor AClass(int) 8
after
before AClass c(g())
constructor AClass(int) 8
after
The compiler is allowed to elide copying in a case like this. This is called Return Value Optimization.
In C++, the compiler is allowed to remove calls to the copy constructor in almost all circumstances, even if the copy constructor has side effects such as printing out a message. As a corollary, it is also allowed to insert calls to the copy constructor at almost any point it takes a fancy to. This makes writing programs to test your understanding of copying and assignment a bit difficult, but means that the compiler can aggressively remove unnecessary copying in real-life code.
This is known as "return value optimisation". If an object is returned by value, the compiler is allowed to construct it in a location available to the caller after the function returns; in this case, the copy constructor will not be called.
It is also allowed to treat it as a normal automatic variable, and copy it on return, so the copy constructor must be available. Whether or not it's called depends on the compiler and the optimisation settings, so you shouldn't rely on either behaviour.
This is called Copy Ellision. The compiler is allowed to ellide copies in virtually any situation. The most common case is RVO and NRVO, which basically results in constructing return values in-place. I'll demonstrate the transformation.
void g (char* memory) {
new (memory) AClass(8);
}
int main () {
char __hidden__variable[sizeof(AClass)];
g(__hidden__variable);
AClass& b = *(AClass*)&__hidden__variable[0];
cout -- " after" -- endl;
// The same process occurs for c.
}
The code has the same effect, but now only one instance of AClass exists.
The compiler may have optimized away the copy constructor call. Basically, it moves the object.
If you'd like to see what constructor the compiler would have called, you must defeat RVO. Replace your g() function thus:
int i;
AClass g () {
if(i) {
AClass x(8);
return x;
} else {
AClass x(9);
return x;
}
}
I was going through Josuttis's "Using Map's as associative arrays" (from The C++ Standard Library - A Tutorial and Reference, 2nd Edition) and came across Using a std::map as an associative array on Stack Overflow. Now I have more questions on the constructors that are called when inserting into a map.
Here is my sample program (not using best coding practices; please excuse me for that):
class C
{
public:
string s;
C() { cout << "default " << endl;}
C(const string& p) : s(p)
{ cout << "one param" << endl;}
C(const C& obj)
{
if (this != &obj)
{
s = obj.s;
}
cout << "copy constr" << endl;
}
C& operator = (const C& obj)
{
if (this != &obj)
{
s = obj.s;
}
cout << "copy initializer" << endl;
return *this;
}
};
int main()
{
map<int,C> map1;
C obj("test");
cout << "Inserting using index" << endl;
map1[1] = obj;
cout << "Inserting using insert / pair" << endl;
map1.insert(make_pair(2,obj));
}
The output for this program is:
one param
Inserting using index
default
copy constr
copy constr
copy initializer
Inserting using insert / pair
copy constr
copy constr
copy constr
copy constr
I was assuming that initializing the map by index should call the default constructor and followed by the assignment operator.
But executing map1[1] = obj creates following output;
Inserting using index
default
copy constr
copy constr
copy initializer
Can someone help me to understand the initialization better?
If you read the specification for std::map, it says that operator[] is equivalent to (in this case)
(*((this->insert(make_pair(1,C()))).first)).second
So this explains all the constructor calls you see. First it calls the default constructor C(). Then it calls make_pair, which copies the C object. Then it calls insert, which makes a copy of the pair object you just made, calling the C copy constructor again. Finally it calls the assignment operator to set the inserted object to the one you are assigning it to.
Don;t know.
But this is interesting:
#include <string>
#include <map>
#include <iostream>
using namespace std;
class C
{
public:
string s;
C()
{
cout << "default " << endl;
}
C(const string& p)
: s(p)
{
cout << "one param(" << s << ")" << endl;
}
C(const C& obj)
:s(obj.s)
{
cout << "copy constr(" << s << ")" <<endl;
}
C& operator = (const C& obj)
{
cout << "copy initializer\t" <<;
C copy(obj);
std::swap(s,copy.s);
return *this;
}
};
int main()
{
map<int,C> map1;
cout << "Inserting using index" << endl;
map1[1] = C("Plop");
}
It looks like the default one is created and copied around.
Then the external one is just assinged over it once it has been put in place.
Inserting using index
default
copy constr()
copy constr()
one param(Plop)
copy initializer copy constr(Plop)
What happens if you simply execute map[1];? This may involve internal copies, depending on the implementation of map your standard library uses.
Actually map1[1] = obj will create pair first