What is the difference between const and constexpr in C++?

Point	const	constexpr
Meaning	Read only after the start	Computed at compile time
Value known	Can be at run time	Must be at compile time
Use for array sizes	Only if the value is a compile time constant	Yes

#include <array>
#include <iostream>

constexpr int square(int n) { return n * n; }

int main() {
    constexpr int size = square(3);               // calculated while compiling
    std::array<int, size> a{};
    const int input = 5;                          // const, set at the start
    std::cout << a.size() << ' ' << input << '\n';
}

See C++ Variables.

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