C++ – constexpr vs. static const: Which One to Prefer?

c++c++11constantsconstexpr

For defining compile-time constants of integral types like the following (at function and class scope), which syntax is best?

static const int kMagic = 64; // (1)
constexpr int kMagic = 64;    // (2)

(1) works also for C++98/03 compilers, instead (2) requires at least C++11. Are there any other differences between the two? Should one or the other be preferred in modern C++ code, and why?

EDIT

I tried this sample code with Godbolt's CE:

int main()
{
#define USE_STATIC_CONST
#ifdef USE_STATIC_CONST
  static const int kOk = 0;
  static const int kError = 1;
#else
  constexpr int kOk = 0;
  constexpr int kError = 1;
#endif
  return kOk;
}

and for the static const case this is the generated assembly by GCC 6.2:

main::kOk:
        .zero   4
main::kError:
        .long   1
main:
        push    rbp
        mov     rbp, rsp
        mov     eax, 0
        pop     rbp
        ret

On the other hand, for constexpr it's:

main:
        push    rbp
        mov     rbp, rsp
        mov     DWORD PTR [rbp-4], 0
        mov     DWORD PTR [rbp-8], 1
        mov     eax, 0
        pop     rbp
        ret

Although at -O3 in both cases I get the same (optimized) assembly:

main:
        xor     eax, eax
        ret

EDIT #2

I tried this simple code (live on Ideone):

#include <iostream>
using namespace std;

int main() {
    const int k1 = 10;
    constexpr int k2 = 2*k1;
    cout << k2 << '\n';
    return 0;
}

which shows that const int k1 is evaluated at compile-time, as it's used to calculate constexpr int k2.

However, there seems to be a different behavior for doubles. I've created a separate question for that here.

Best Answer

A constexpr variable is guaranteed to have a value available at compile time, whereas static const members or const variables could either mean a compile time value or a runtime value. constexpr expresses your intent of a compile time value in a much more explicit way than const.

One more thing: in C++17, constexpr static data member variables will be inline too. That means you can omit the out of line definition of static constexpr variables, but not of static const.

As a demand in the comment section, here's a more detailed explanation about static const in function scope.

A static const variable at function scope is pretty much the same, but instead of having an automatic storage duration, it has static storage duration. That mean it's in some way the equivalent of declaring the variable as global, but only accessible in the function.

It is true that a static variable is initialized at the first call of the function, but since it's const too, the compiler will try to inline the value and optimize out the variable completely. So in a function, if the value is known at compile time for this particular variable, then the compiler will most likely optimize it out.

However, if the value isn't known at compile time for a static const at function scope, it might silently make your function (a very small bit) slower, since it has to initialize the value at runtime the first time the function is called. Plus, it has to check if the value is initialized each time the function is called.

That's the advantage of a constexpr variable. If the value isn't known at compile time, it's a compilation error, not a slower function. Then if you have no way of determine the value of your variable at compile time, then the compiler will tell you about that and you can do something about it.

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