The C++ Core Guidelines recommend using lambda initialization for complex initialization: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-lambda-init

It gives the following reason:

Reason It nicely encapsulates local initialization, including cleaning up scratch variables needed only for the initialization, without needing to create a needless nonlocal yet nonreusable function. It also works for variables that should be const but only after some initialization work.

These examples use GCC 9.1 on x86-64 using -O2.

The following program with lambda initialization:

#include <ctime>

int func() {
    return std::time(nullptr);
}

int main() {
    const int x = std::time(nullptr);
    const int a = [&]() {
        const int e = func() + x * func();
        const int f = func() * func();
        return e + f;
    }();
    return a;
}

Compiles to the following assembly:

func():
        sub     rsp, 8
        xor     edi, edi
        call    time
        add     rsp, 8
        ret
main:
        push    r12
        xor     edi, edi
        push    rbx
        sub     rsp, 8
        call    time
        xor     edi, edi
        mov     r12, rax
        call    time
        xor     edi, edi
        mov     rbx, rax
        call    time
        xor     edi, edi
        imul    r12d, eax
        call    time
        xor     edi, edi
        add     r12d, ebx
        mov     rbx, rax
        call    time
        add     rsp, 8
        imul    ebx, eax
        lea     eax, [rbx+r12]
        pop     rbx
        pop     r12
        ret

The following program without lambda initialization:

#include <ctime>

int func() {
    return std::time(nullptr);
}

int main() {
    const int x = std::time(nullptr);
    const int e = func() + x * func();
    const int f = func() * func();
    const int a = e + f;
    return a;
}

Compiles to the following assembly:

func():
        sub     rsp, 8
        xor     edi, edi
        call    time
        add     rsp, 8
        ret
main:
        push    r12
        xor     edi, edi
        push    rbp
        sub     rsp, 8
        call    time
        xor     edi, edi
        mov     rbp, rax
        call    time
        xor     edi, edi
        mov     r12, rax
        call    time
        xor     edi, edi
        imul    ebp, eax
        call    time
        xor     edi, edi
        add     r12d, ebp
        mov     rbp, rax
        call    time
        add     rsp, 8
        imul    eax, ebp
        pop     rbp
        add     eax, r12d
        pop     r12
        ret