Compiling with gcc-7.1 with the flag -std=c++17
, the following program raises an error:
#include <string_view>
void foo(const char* cstr) {}
void bar(std::string_view str){
foo(str);
}
The error message is
In function 'void bar(std::string_view)':
error: cannot convert 'std::string_view {aka std::basic_string_view<char>}' to 'const char*' for argument '1' to 'void foo(const char*)'
foo(str);
I'm surprised there is no conversion to const char*
because other libraries (abseil, bde), provide similar string_view
classes which implicitly convert to const char*
.
A std::string_view
doesn't provide a conversion to a const char*
because it doesn't store a null-terminated string. It stores a pointer to the first element, and the length of the string, basically. That means that you cannot pass it to a function expecting a null-terminated string, like foo
(how else are you going to get the size?) that expects a const char*
, and so it was decided that it wasn't worth it.
If you know for sure that you have a null-terminated string in your view, you can use std::string_view::data
.
If you're not you should reconsider whether using a std::string_view
in the first place is a good idea, since if you want a guaranteed null-terminated string std::string
is what you want. For a one-liner you can use std::string(object).c_str()
(note: the return value points to a temporary std::string
instance that will get destroyed after the end of the expression!).