The strings topic in the SO Documentation used to say, in the Remarks section:
Since C++14, instead of using
"foo"
, it is recommended to use"foo"s
, ass
is a string literal, which converts theconst char *
"foo"
tostd::string
"foo"
.
The only advantage I see using
std::string str = "foo"s;
instead of
std::string str = "foo";
is that in the first case the compiler can perform copy-elision (I think), which would be faster than the constructor call in the second case.
Nonetheless, this is (not yet) guaranteed, so the first one might also call a constructor, the copy constructor.
Ignoring cases where it is required to use std::string
literals like
std::string str = "Hello "s + "World!"s;
is there any benefit of using std::string
literals instead of const char[]
literals?
Definition. String literal in Java is a set of characters that is created by enclosing them inside a pair of double quotes. In contrast, String Object is a Java is a set of characters that is created using the new() operator. Thus, this explains the main difference between string literal and string object.
In source code, any literal, whether user-defined or not, is essentially a sequence of alphanumeric characters, such as 101 , or 54.7 , or "hello" or true . The compiler interprets the sequence as an integer, float, const char* string, and so on.
A string literal is what you type into your source code. The string value is what gets output when you print it.
C-strings are simply implemented as a char array which is terminated by a null character (aka 0 ). This last part of the definition is important: all C-strings are char arrays, but not all char arrays are c-strings. C-strings of this form are called “string literals“: const char * str = "This is a string literal.
If you're part of the "Almost Always Auto" crowd, then the UDL is very important. It lets you do this:
auto str = "Foo"s;
And thus, str
will be a genuine std::string
, not a const char*
. It therefore permits you to decide when to do which.
This is also important for auto return type deduction:
[]() {return "Foo"s;}
Or any form of type deduction, really:
template<typename T> void foo(T &&t) {...} foo("Foo"s);
The only advantage I see using [...] instead of [...] is that in the first case the compiler can perform copy-elision (I think), which would be faster than the constructor call in the second case.
Copy-elision is not faster than the constructor call. Either way, you're calling one of the object's constructors. The question is which one:
std::string str = "foo";
This will provoke a call to the constructor of std::string
which takes a const char*
. But since std::string
has to copy the string into its own storage, it must get the length of the string to do so. And since it doesn't know the length, this constructor is forced to use strlen
to get it (technically, char_traits<char>::length
, but that's probably not going to be much faster).
By contrast:
std::string str = "foo"s;
This will use the UDL template that has this prototype:
string operator "" s(const char* str, size_t len);
See, the compiler knows the length of a string literal. So the UDL code is passed a pointer to the string and a size. And thus, it can call the std::string
constructor that takes a const char*
and a size_t
. So there's no need for computing the string's length.
The advice in question is not for you to go around and convert every use of a literal into the s
version. If you're fine with the limitations of an array of char
s, use it. The advice is that, if you're going to store that literal in a std::string
, it's best to get that done while it's still a literal and not a nebulous const char*
.
The advice to use "blah"s
has nothing to do with efficiency and all to do with correctness for novice code.
C++ novices who don't have a background in C, tend to assume that "blah"
results in an object of some reasonable string type. For example, so that one can write things like "blah" + 42
, which works in many script languages. With "blah" + 42
in C++, however, one just incurs Undefined Behavior, addressing beyond the end of the character array.
But if that string literal is written as "blah"s
then one instead gets a compilation error, which is much preferable.
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