how do static variables inside functions work?

Question

In the following code:

int count(){
    static int n(5);
    n = n + 1;
    return n;
}

the variable n is instantiated only once a

Answer 1

This is, of course, compiler-specific.

The reason you didn't see any checks in the generated assembly is that, since n is an int variable, g++ simply treats it as a global variable pre-initialized to 5.

Let's see what happens if we do the same with a std::string:

#include 

void count() {
    static std::string str;
    str += ' ';
}

The generated assembly goes like this:

_Z5countv:
.LFB544:
        .cfi_startproc
        .cfi_personality 0x3,__gxx_personality_v0
        .cfi_lsda 0x3,.LLSDA544
        pushq   %rbp
        .cfi_def_cfa_offset 16
        movq    %rsp, %rbp
        .cfi_offset 6, -16
        .cfi_def_cfa_register 6
        pushq   %r13
        pushq   %r12
        pushq   %rbx
        subq    $8, %rsp
        movl    $_ZGVZ5countvE3str, %eax
        movzbl  (%rax), %eax
        testb   %al, %al
        jne     .L2                     ; <======= bypass initialization
        .cfi_offset 3, -40
        .cfi_offset 12, -32
        .cfi_offset 13, -24
        movl    $_ZGVZ5countvE3str, %edi
        call    __cxa_guard_acquire     ; acquire the lock
        testl   %eax, %eax
        setne   %al
        testb   %al, %al
        je      .L2                     ; check again
        movl    $0, %ebx
        movl    $_ZZ5countvE3str, %edi
.LEHB0:
        call    _ZNSsC1Ev               ; call the constructor
.LEHE0:
        movl    $_ZGVZ5countvE3str, %edi
        call    __cxa_guard_release     ; release the lock
        movl    $_ZNSsD1Ev, %eax
        movl    $__dso_handle, %edx
        movl    $_ZZ5countvE3str, %esi
        movq    %rax, %rdi
        call    __cxa_atexit            ; schedule the destructor to be called at exit
        jmp     .L2
.L7:
.L3:
        movl    %edx, %r12d
        movq    %rax, %r13
        testb   %bl, %bl
        jne     .L5
.L4:
        movl    $_ZGVZ5countvE3str, %edi
        call    __cxa_guard_abort
.L5:
        movq    %r13, %rax
        movslq  %r12d,%rdx
        movq    %rax, %rdi
.LEHB1:
        call    _Unwind_Resume
.L2:
        movl    $32, %esi
        movl    $_ZZ5countvE3str, %edi
        call    _ZNSspLEc
.LEHE1:
        addq    $8, %rsp
        popq    %rbx
        popq    %r12
        popq    %r13
        leave
        ret
        .cfi_endproc

The line I've marked with the bypass initialization comment is the conditional jump instruction that skips the construction if the variable already points to a valid object.