Why doesn't the STL string class have an overloaded char* operator built-in? Is there any specific reason for them to avoid it?
If there was one, then using the string class with C functions would become much more convenient.
I would like to know your views.
Following is the quote from Josuttis STL book:
However, there is no automatic type
conversion from a string object to a
C-string. This is for safety reasons
to prevent unintended type conversions
that result in strange behavior (type
char* often has strange behavior) and
ambiguities (for example, in an
expression that combines a string and
a C-string it would be possible to
convert string into char* and vice
versa). Instead, there are several
ways to create or write/copy in a
C-string, In particular, c_str() is
provided to generate the value of a
string as a C-string (as a character
array that has '\0' as its last
character).
You should always avoid cast operators, as they tend to introduce ambiguities into your code that can only be resolved with the use of further casts, or worse still compile but don't do what you expect. A char*() operator would have lots of problems. For example:
string s = "hello";
strcpy( s, "some more text" );
would compile without a warning, but clobber the string.
A const version would be possible, but as strings must (possibly) be copied in order to implement it, it would have an undesirable hidden cost. The explicit c_str() function means you must always state that you really intend to use a const char *.
The string template specification deliberately allows for a "disconnected" representation of strings, where the entire string contents is made up of multiple chunks. Such a representation doesn't allow for easy conversion to char*.
However, the string template also provides the c_str method for precisely the purpose you want: what's wrong with using that method?
By 1998-2002 it was hot topic of c++ forums. The main problem - zero terminator. Spec of std::?string allows zero character as normal, but char* string doesn't.
You can use c_str instead:
string s("I like rice!");
const char* cstr = s.c_str();
I believe that in most cases you don't need the char*, and can work more conveniently with the string class itself.
If you need interop with C-style functions, using a std::vector<char> / <wchar_t> is often easier.
It's not as convenient, and unfortunately you can't O(1)-swap it with a std::string (now that would be a nice thing).
In that respect, I much prefer the interface of MFC/ATL CString which has stricter performance guarantees, provides interop, and doesn't treat wide character/unicode strings as totally foreign (but ok, the latter is somewhat platform specific).
Related
Note:
I heavily changed my question to be more specific, but I will keep the old question at end of the post, in case it is useful to anyone.
New Question
I am developing an embedded application which uses the following types to represent strings :
string literals(null terminated by default)
std::array<char,size> (not null terminated)
std::string_view
I would like to have a function that accepts all of them in a uniform way. The only problem is that if the input is a string literal I will have to count the size with strlen that in both other cases doesn't work but if I use size it will not work on case 1.
Should I use a variant like so: std::variant<const char *,std::span<char>> ? Would that be heavy by forcing myself to use std::visit ? Would that thing even match correctly all the different representations of strings?
Old Question
Disclaimer when I refer to "string" in the following context I don't mean an std::string but just an abstract way to say alphanumeric series.
Most of the cases when I have to deal with strings in c++ I use something like void func(const std::string &); or without the const and the reference at some cases.Now on an embedded app I don't have access to std::string and I tried to use std::string_view the problem is that std::string_view when constructed from a non literal sometimes is not null terminated
Edit: I changed the question a bit as the comments implied some very helphull hints .
So even though y has a size in the example below:
std::array<char,5> x{"aa"} ;
std::string_view y(x.data());
I can't use y with a c api like printf(%s,y.data()) that is based on null termination
#include <array>
#include <string_view>
#include "stdio.h"
int main(){
std::array<char,5> x{"aaa"};
std::string_view y(x.data());
printf("%s",x);
}
To summarize:
What can I do to implement a stack allocated string that implicitly gets a static size at its constructors (from null terminated strings,string literals, string_views and std::arrays) and it is movable (or cheap copyable)?
What would be the underlying type of my class? What would be the speed costs in comparison with the underlying type?
I think that you are looking at two largely and three subtly different semantics of char*.
Yes, all of them point at char but the type-specific info on how to determine the length is not carried by that. Even in the ancient ancestor of C++ (not saying C...) a pointer to char was not always the same. Already there pointers to terminated and non-terminated sequences of characters could not be mixed.
In C++ the tool of overloading a function exists and it seems to be the obvious solution for your problem. You can still implement that efficiently with only one (helper) function doing the actual work, based on an explicit size information in a second parameter.
Overload the function which is "visible" on the API, with three versions for the three types. Have it determine the length in the appropriate way, then call the single helper function, providing that length.
One thing that C++ has is multiple string, or rather character types: char, wchar_t, char16_t, char32_t. As a result we have different string typedefs: std::string, std::wstring, std::u16string and std::u32string, which are distinct string types.
And it doesn't stop there, if we are talking Windows and COM, there's also platform types, like BSTRs. And we haven't even started talking about
character encodings.
If you are building a new library, and one of the requirements was to support all those string types, or character types, how would you do it? Let's forget about character encodings for now.
I was thinking about this, and I came out with a few options, but none are ideal. Let's assume you have a registry_key class, which has to
support all those character types, and a part of its OM is more or less (only a part of it is illustrated here):
class registry_key
{
public:
registry_key(unspecified_string_type keyname);
unspecified_string_type name() const;
unspecified_string_type path() const;
}
And you would use it like:
registry_key key("HKLM\\Software\\Adobe");
std::string name = key.name();
But, it has to support the other string types. Also, there isn't a requirement which dictates that the whole registry_key has to be consistent as far as character types go, or operate on a single character type. You could call the constructor and pass a const char* but get the name of the key as a u16string. This is a reflection of the platform underneath, which allows you to call wide (XxxW) and narrow (XxxA) apis within the same api set. And that behavior is desired.
For the constructor (or things taking arguments) this is trivial, because the type can be deduced. But not for a function that returns strings but doesn't take anything as a input, it can't.
As far as options go, I have:
1) Template the whole registry key with a character type, the same way basic_string, and other types in the stl did. So you would
wregistry_key key(L"HKLM\\Software\\Adobe");
std::wstring name = key.name();
u8registry_key key(u8"HKLM\\Software\\Adobe");
std::u16string name = key.name();
The problem is that this doesn't really scale and it's quite horrible if it has to be applied to a lot of types, anything which deals with strings. And in a way it's a poor design choice because some classes aren't even about strings so much, so why pass that in as a template argument in the first place.
2) Adopt and use a single string type, like u16string, or u32string. But as said, that is against the goal.
3) Prefix the character type to the function names:
registry_key key("HKLM\\Software\\Adobe");
std::string name = key.name();
std::wstring name = key.wname();
std::u16string name = key.u8name();
std::u32string name = key.uname();
This is better, but still redundand.
4) Make a new string type, which isn't a string type at all. It's in a way a variant that can store different types of strings, and queried and converted to different other types of strings, using user defined conversion operators. So this would be automatic.
platform_string str = L"foo";
std::string sstr = str;
std::wstring swstr = str;
std::u16string su16str = str;
str = u"foo";
This would enable writing a registry class which could looks like this:
class registry_key
{
public:
registry_key(unspecified_string_type keyname);
platform_string name() const;
platform_string path() const;
}
And you could use it as:
registry_key key("HKLM\\Software\\Adobe");
std::string name = key.name();
std::wstring name = key.name();
std::u16string name = key.name();
The problem with this is the idea of introducing something that looks like a new string type, even if it isn't really. And it feels broken.
Are there better solutions than 3) and 4)? Or a better way to solve this problem?
The customary approach to this type of problem is for the library designer to pick one type of string and use it consistently throughout its interfaces. If you need C compatibility, use a C format string, otherwise a C++ string. Pick the character size you need for the library's functions.
Let the caller of the library handle the string conversion.
Otherwise, you are going to have a mess.
If you are building a new library, and one of the requirements was to support all those string types, or character types, how would you do it?
I wouldn't.
Between std::codecvt and boost::nowide, conversion between the various string formats isn't totally onerous these days.
I'd probably just use UTF-32 internally (RAM is cheap these days, right?) and a UTF-8 public interface. A UTF-16 (using char16_t, not wchar_t) public interface might also be justifiable seeing as so many platforms use it internally, especially Windows, though I'd rather avoid such a thing (unless I was being paid for it, perhaps).
wstring and wchar_t should be avoided wherever possible because of portability issues, as the definition of wchar_t is platform dependent. Only character types with explicit widths should be used (eg. char, char16_t and char32_t).
Your option (3) involves quadrupling the size of your API... no thanks!
Your option (4) feels like it would be exceptionally difficult to do well, adding a big chunk of complexity in exchange for a modicum of convenient.
Forcing the caller to do string conversion seems like the simplest, safest and maximally portable way to handle the problem. Solution (2) all the way.
Oh, and the usual http://utf8everywhere.org/ link for completeness.
Even though I'd go with others' suggestions and have my library deal with one string type, I'd urge you to have a look at boost.filesystem, in particular the design of the path class
Boost Filesystem V3 design. Basically you have no template class, internally you use just one string type, and then provide templated members that accept whatever string type you decide and convert to the internal representation.
This is a follow up of this question. Suppose I write a C++ interface that accepts or returns a const string. I can use a const char* zero-terminated string:
void f(const char* str); // (1)
The other way would be to use an std::string:
void f(const string& str); // (2)
It's also possible to write an overload and accept both:
void f(const char* str); // (3)
void f(const string& str);
Or even a template in conjunction with boost string algorithms:
template<class Range> void f(const Range& str); // (4)
My thoughts are:
(1) is not C++ish and may be less efficient when subsequent operations may need to know the string length.
(2) is bad because now f("long very long C string"); invokes a construction of std::string which involves a heap allocation. If f uses that string just to pass it to some low-level interface that expects a C-string (like fopen) then it is just a waste of resources.
(3) causes code duplication. Although one f can call the other depending on what is the most efficient implementation. However we can't overload based on return type, like in case of std::exception::what() that returns a const char*.
(4) doesn't work with separate compilation and may cause even larger code bloat.
Choosing between (1) and (2) based on what's needed by the implementation is, well, leaking an implementation detail to the interface.
The question is: what is the preffered way? Is there any single guideline I can follow? What's your experience?
Edit: There is also a fifth option:
void f(boost::iterator_range<const char*> str); // (5)
which has the pros of (1) (doesn't need to construct a string object) and (2) (the size of the string is explicitly passed to the function).
If you are dealing with a pure C++ code base, then I would go with #2, and not worry about callers of the function that don't use it with a std::string until a problem arises. As always, don't worry too much about optimization unless there is a problem. Make your code clean, easy to read, and easy to extend.
There is a single guideline you can follow: use (2) unless you have very good reasons not to.
A const char* str as parameter does not make it explicit, what operations are allowed to be performed on str. How often can it be incremented before it segfaults? Is it a pointer to a char, an array of chars or a C string (i.e. a zero-terminated array of char)?
I don't really have a single hard preference. Depending on circumstances, I alternate between most of your examples.
Another option I sometimes use is similar to your Range example, but using plain old iterator ranges:
template <typename Iter>
void f(Iter first, Iter last);
which has the nice property that it works easily with both C-style strings (and allows the callee to determine the length of the string in constant time) as well as std::string.
If templates are problematic (perhaps because I don't want the function to be defined in a header), I sometimes do the same, but using char* as iterators:
void f(const char* first, const char* last);
Again, it can be trivially used with both C-strings and C++ std::string (as I recall, C++03 doesn't explicitly require strings to be contiguous, but every implementation I know of uses contiguously allocated strings, and I believe C++0x will explicitly require it).
So these versions both allow me to convey more information than the plain C-style const char* parameter (which loses information about the string length, and doesn't handle embedded nulls), in addition to supporting both of the major string types (and probably any other string class you can think of) in an idiomatic way.
The downside is of course that you end up with an additional parameter.
Unfortunately, string handling isn't really C++'s strongest side, so I don't think there is a single "best" approach. But the iterator pair is one of several approaches I tend to use.
For taking a parameter I would go with whatever is simplest and often that is const char*. This works with string literals with zero cost and retrieving a const char* from something stored in a std:string is typically very low cost as well.
Personally, I wouldn't bother with the overload. In all but the simplest cases you will want to merge to two code paths and have one call the other at some point or both call a common function. It could be argued that having the overload hides whether one is converted to the other or not and which path has a higher cost.
Only if I actually wanted to use const features of the std::string interface inside the function would I have const std::string& in the interface itself and I'm not sure that just using size() would be enough of a justification.
In many projects, for better or worse, alternative string classes are often used. Many of these, like std::string give cheap access to a zero-terminated const char*; converting to a std::string requires a copy. Requiring a const std::string& in the interface is dictating a storage strategy even when the internals of the function don't need to specify this. I consider it this to be undesirable, much like taking a const shared_ptr<X>& dictates a storage strategy whereas taking X&, if possible, allows the caller to use any storage strategy for a passed object.
The disadvantages of a const char* are that, purely from an interface standpoint, it doesn't enforce non-nullness (although very occasionally the difference betweem a null parameter and an empty string is used in some interfaces - this can't be done with std::string), and a const char* might be the address of just a single character. In practice, though, the use of a const char* to pass a string is so prevalent that I would consider citing this as a negative to be a fairly trivial concern. Other concerns, such as whether the encoding of the characters specified in the interface documentation (applies to both std::string and const char*) are much more important and likely to cause more work.
The answer should depend heavily on what you are intending to do in f. If you need to do some complex processing with the string, the approach 2 makes sense, if you simply need to pass to some other functions, then select based on those other functions (let's say for arguments sake you are opening a file - what would make most sense? ;) )
It's also possible to write an
overload and accept both:
void f(const string& str) already accepts both because of the implicit conversion from const char* to std::string. So #3 has little advantage over #2.
I would choose void f(const string& str) if the function body does not do char-analysis; means it's not referring to char* of str.
Use (2).
The first stated problem with it is not an issue, because the string has to be created at some point regardless.
Fretting over the second point smells of premature optimization. Unless you have a specific circumstance where the heap allocation is problematic, such as repeated invocations with string literals, and those cannot be changed, then it is better to favor clarity over avoiding this pitfall. Then and only then might you consider option (3).
(2) clearly communicates what the function accepts, and has the right restrictions.
Of course, all 5 are improvements over foo(char*) which I have encountered more than I would care to mention.
I'm a bit fuzzy on the basic ways in which programmers code differently in C and C++. One thing in particular is the usage of strings in C++ over char arrays, or vice versa. So, should I use strings or char arrays, in general, and why?
In C++ you should in almost all cases use std::string instead of a raw char array.
std::string manages the underlying memory for you, which is by itself a good enough reason to prefer it.
It also provides a much easier to use and more readable interface for common string operations, e.g. equality testing, concatenation, substring operations, searching, and iteration.
If you're modifying or returning the string, use std::string. If not, accept your parameter as a const char* unless you absolutely need the std::string member functions. This makes your function usable not only with std::string::c_str() but also string literals. Why make your caller pay the price of constructing a std::string with heap storage just to pass in a literal?
Others have put it. Use the std::string stuff wherever possible. However there are areas where you need char *, e.g if you like to call some system-services.
As is the case with everything what you choose depends on what you're doing with it. std::string has real value if you're dealing with string data that changes. You can't beat char[] for efficiency when dealing with unchanging strings.
Use std::string.
You will have less problems (I think almost none, at least none come to my mind) with buffer sizes.
C has char[] while c++ has std::string too...
I commonly hear that one should "Embrace the language" and, following that rule, you should use std::string...
However, its pretty much up to what library are you using, how does that library want you to express your strings, stuff like that.
std::string is a container class, and inside it, is a char[]
If you use std::string, you have many advantages, such as functions that will help you [compare, substr, as examples]
I often see the following structure, especially in constructors:
class::class(const string &filename)
{
}
class::class(const char * const filename)
{
}
By step-by-step debug, I found out the 2nd constructor is always called if I pass a hard-coded string.
Any idea:
1) Why the dual structure is used?
2) What is the speed difference?
Thanks.
Two constructors are needed because you can pass a NULL to your MyClass::MyClass(const std::string &arg). Providing second constructor saves you from a silly crash.
For example, you write constructor for your class, and make it take a const std::string & so that you don't have to check any pointers to be valid if you'd be using const char*.
And everywhere in your code you're just using std::strings. At some point you (or another programmer) pass there a const char*. Here comes nice part of std::string - it has a constructor, which takes char*, and that's very good, apart from the fact, that std::string a_string(NULL) compiles without any problems, just doesn't work.
That's where a second constructor like you've shown comes handy:
MyClass::MyClass(const char* arg)
: m_string(arg ? arg : "")
{}
and it will make a valid std::string object if you pass it a NULL.
In this case I don't think you'd need to worry about any speed. You could try measuring, although I'm afraid you'd be surprised with how little difference (if any) there would be.
EDIT: Just tried std::string a_string(NULL);, compiles just fine, and here's what happens when it is run on my machine (OS X + gcc 4.2.1) (I do recall I tried it on Windows some time ago, result was very similar if not exactly same):
std::logic_error: basic_string::_S_construct NULL not valid
This is useful if the implementation deals with const char*s by itself, but is mostly called by std::string users. These can call using the std::string API, which usually just calls c_str() and dispatches to the const char* implementation. On the other side, if the caller does already have a c-string, no temporary or unneeded std::string needs to be constructed (which can be costly, for longer strings it's a heap allocation).
Also, I once used it to resolve the following case:
My interface took std::string's, but had to be implemented in an external module, thus the STL binary versions of both the module AND the caller module had to match exactly, or it would have crashed (not really good for a portable library… ). So I changed the actual interface to use const char*, and added std::string overloads which I declared inline, so they weren't exported. It didn't break existing code, but resolved all my module boundary problems.
1) Why the dual structure is used?
The string reference version is required if std::string objects are to be used conveniently as parametersm as there is no implicit conversion from a std::string to a const char const. The const char * const version is optional, as character arrays can implicitly be converted into std::strings, but it is more efficient, as no temporary std::string need be created.
2) What is the speed difference?
You will need to measure that yourself.
They are offered basically for convenience. Some times, if you call C functions, you get char* pointers. Others, you get strings, so offering both constructors is just a convenience for the caller. As for the speed, both have virtually the same speed, as they both send a memory address to the constructor.