Question: Why is return ""; faster than return {}; when initializing an empty std::string?
To answer this question, we can look at the generated assembly code when compiling the two functions, make_default() and make_empty():
#include <string>
std::string make_default() {
return {};
}
std::string make_empty() {
return "";
}
Compiling the two functions with Clang 16 and libc++ produces the following assembly code:
make_default():
mov rax, rdi
xorps xmm0, xmm0
movups xmmword ptr [rdi], xmm0
mov qword ptr [rdi + 16], 0
ret
make_empty():
mov rax, rdi
mov word ptr [rdi], 0
ret
See live example at Compiler Explorer.
The assembly code for make_default() zeroes out 24 bytes in the std::string container, while make_empty() only zeroes out 2 bytes. This is why return ""; is faster than return {}; when creating an empty std::string.