What are 'constexpr' useful for?

Question

I really can\'t find any use of it. My first idea was that I could use it to implement \'Design by Contract\' without using macros like this:

struct S
{   
    S         


        

Answer 1

Answer of " Can someone give me some insight why constexpr keyword was being introduced?"

Modern C++ supports two types of immutability.

1) const

2) constexpr.

constexper will be evaluated at the compile time. It is used to specify constness and allows placement of data in the memory where it might be corrupted.

Example 1:

void UseConstExpr(int temp)
{
    // This code snippet is OK
    constexpr int y = max + 300;
    std::cout << y << std::endl;
    // This code snippet gives compilation error
    // [ expression must have a constant value]
    constexpr int z = temp + 300;

}

Example 2:

int addVector(const std::vector& inVect)
{
    int sum = 0;
    for ( auto& vec : inVect)
    {
        sum += vec;
    }
    std::cout << sum << std::endl;
    return sum;
}

int main()
{
    // vInt is not constant
    std::vector vInt = { 1,2,3,4,5,6 }; 
    // This code snippet is OK
    // because evaluated at run time
    const int iResult = addVector(vInt);
    // Compiler throws below error
    // function call must have a constant value in a constant expression
    // because addVector(vInt) function is not a constant expression
    constexpr int iResult = addVector(vInt);
    return 0;
}

Note: above source code are compiled on VS2015