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

Here is a summary of points made by Alex Allain in his "Constexpr - Generalized Constant Expressions in C++11," which details the usefulness of constexpr:

• First, with a constexpr specifier, the value of the function or variable can be at compile time.
• Another benefit of the constexpr specifier is that it can replace macros with functions
• constexpr will also benefit your template metaprogramming.

Benefit to efficiency:

constant expressions...allow certain computations to take place at compile time, literally while your code compiles rather than when the program itself is run. (Allain 2)

Performance benefit: if something can be done at compile time, it will be done once, rather than every time the program runs

Other benefits:

Such variables and functions can then be used where only compile time constant expressions are allowed. A constexpr specifier used in an object declaration implies const. A constexpr specifier used in an function declaration implies inline.(CPP 1)

Rules for constexpr functions:

1. It must consist of single return statement (with a few exceptions)
2. It can call only other constexpr functions
3. It can reference only constexpr global variables (Allain 6)

Rules for constexpr constructors:

1. each of its parameters must be literal type
2. the class must have no virtual base classes
3. the constructor must not have a function-try-block (CPP 6)

Citations

Allain, Alex, "Constexpr - Generalized Constant Expressions in C++11", Unspecified Date, "http://www.cprogramming.com/c++11/c++11-compile-time-processing-with-constexpr.html"

CPP, "Constexpr Specifier", 16 December 2014, http://en.cppreference.com/w/cpp/language/constexpr

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EDIT: Sorry, It was my fault to make it seem like I was the author of these points, so I have corrected myself, changed various parts, and added citations to avoid plagiarism.