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C++ `std::string::empty`

There are a few common ways check if a C++ std::string is empty (i.e. containing no characters):

s.empty()
s == ""
s == std::string()
s.length() == 0 and s.size() == 0

Let's compare some implementations of std::string.

Assembly showdown

Code generated for different `std::string` implementations and compilers
Library	Compiler	`s.empty()`	`s.size() == 0`	`s == std::string()`	`s == ""`
libstdc++	GCC 9.3 x86_64 `-O2`	`is_string_empty: cmp QWORD PTR [rdi+8], 0 sete al ret`			`.LC0: .string "" is_string_empty: sub rsp, 8 mov esi, OFFSET FLAT:.LC0 call std::string::compare test eax, eax sete al add rsp, 8 ret`
libstdc++	Clang 10.0.0 x86_64 `-O2`	`is_string_empty: cmp QWORD PTR [rdi+8], 0 sete al ret`			`is_string_empty: push rax mov esi, OFFSET .L.str call std::string::compare test eax, eax sete al pop rcx ret .L.str: .zero 1`
Library	Compiler	`s.empty()`	`s.size() == 0`	`s == std::string()`	`s == ""`
libc++	Clang 10.0.0 x86_64 `-O2`	`is_string_empty: movzx eax, BYTE PTR [rdi] test al, 1 je .LBB0_1 mov rax, [rdi+8] jmp .LBB0_3 .LBB0_1: shr rax .LBB0_3: test rax, rax sete al ret`			`is_string_empty: push rax movzx eax, BYTE PTR [rdi] test al, 1 je .LBB0_4 mov rax, [rdi+8] test rax, rax je .LBB0_5 .LBB0_2: xor eax, eax pop rcx ret .LBB0_4: shr rax test rax, rax jne .LBB0_2 .LBB0_5: mov ecx, OFFSET .L.str xor esi, esi mov rdx, -1 xor r8d, r8d call std::string::compare test eax, eax sete al pop rcx ret mov rdi, rax call __clang_call_terminate`
libc++	Clang 10.0.0 x86_64 `-O2 -fno-exceptions`				`is_string_empty: push rax movzx eax, BYTE PTR [rdi] test al, 1 je .LBB0_4 mov rax, [rdi+8] test rax, rax je .LBB0_5 .LBB0_2: xor eax, eax pop rcx ret .LBB0_4: shr rax test rax, rax jne .LBB0_2 .LBB0_5: mov ecx, OFFSET .L.str xor esi, esi mov rdx, -1 xor r8d, r8d call std::string::compare test eax, eax sete al pop rcx ret .L.str: .zero 1`
Library	Compiler	`s.empty()`	`s.size() == 0`	`s == std::string()`	`s == ""`
STL	MSVC v19.24 x64 `/O2t`	`is_string_empty: cmp QWORD PTR [rcx+16], 0 sete al ret 0`		`$T1 = 8 __$ArrayPad$ = 40 s$ = 64 is_string_empty: sub rsp, 56 mov rax, __security_cookie xor rax, rsp mov __$ArrayPad$[rsp], rax cmp QWORD PTR [rcx+16], 0 mov QWORD PTR $T1[rsp+16], 0 mov QWORD PTR $T1[rsp+24], 15 mov BYTE PTR $T1[rsp], 0 jne SHORT $LN29@is_string_ mov al, 1 mov rcx, __$ArrayPad$[rsp] xor rcx, rsp call __security_check_cookie add rsp, 56 ret 0 $LN29@is_string_: xor al, al mov rcx, __$ArrayPad$[rsp] xor rcx, rsp call __security_check_cookie add rsp, 56 ret 0`	`is_string_empty: cmp QWORD PTR [rcx+16], 0 jne SHORT $LN16@is_string_ mov al, 1 ret 0 $LN16@is_string_: xor al, al ret 0`
STL	MSVC v19.24 x64 `/O2t /GS-`	`is_string_empty: cmp QWORD PTR [rcx+16], 0 sete al ret 0`		`$T1 = 0 s$ = 48 is_string_empty: $LN63: sub rsp, 40 cmp QWORD PTR [rcx+16], 0 mov QWORD PTR $T1[rsp+16], 0 mov QWORD PTR $T1[rsp+24], 15 mov BYTE PTR $T1[rsp], 0 jne SHORT $LN29@empty_by_c mov al, 1 add rsp, 40 ret 0 $LN29@empty_by_c: xor al, al add rsp, 40 ret 0`
Library	Compiler	`s.empty()`	`s.size() == 0`	`s == std::string()`	`s == ""`

Observations and analysis

For each tested compiler and for each tested library, the code for s.empty() and s.size() == 0 is identical.
MSVC's code for s == "" algorithmically the same as its code for s.empty(), but the implementation uses branches instead of using the sete instruction.
MSVC's implementation and compiler out-performs all other tested compilers in terms of the size of the generated code for s == "", s.empty(), and s == "".

Summary of algorithms for different `std::string` implementations
`std::string` library	`s.empty()`	`s.size() == 0`	`s == std::string()`	`s == ""`
libstdc++	load size and test			call `std::string::compare` (not inlined)
libc++	load and test SSO discriminator; branch and test size			load and test SSO discriminator; branch and test size; `std::string::compare` (not inlined)
STL	load size and test		load size and test; dead stores to stack; branch	load size and test; branch