Using Constant Expressions

C++11 introduces a keyword called constexpr. This is applied to an expression, and indicates that the expression should be evaluated at compile type rather than at runtime:

    constexpr double pi = 3.1415; 
    constexpr double twopi = 2 * pi;

This is similar to initializing a constant declared with the const keyword. However, the constexpr keyword can also be applied to functions that return a value that can be evaluated at compile time, and so this allows the compiler to optimize the code:

    constexpr int triang(int i) 
    { 
       return (i == 0) ? 0 : triang(i - 1) + i;
    }

In this example, the function triang calculates triangular numbers recursively. The code uses the conditional operator. In the parentheses, the function parameter is tested to see if it is zero, and if so the function returns zero, in effect ending the recursion and returning the function to the original caller. If the parameter is not zero, then the return value is the sum of the parameter and the return value of triang called with the parameter is decremented.

This function, when called with a literal in your code, can be evaluated at compile time. The constexpr is an indication to the compiler to check the usage of the function to see if it can determine the parameter at compile time. If this is the case, the compiler can evaluate the return value and produce code more efficiently than by calling the function at runtime. If the compiler cannot determine the parameter at compile-time, the function will be called as normal. A function marked with the constexpr keyword must only have one expression (hence the use of the conditional operator ?: in the triang function).