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Altough it is more thoughly explained in the "Constexpr - Generalized Constant Expressions in C++11", Alex Allain Article, I will simplify it more with a couple simple points.

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

Simple explanation of Usefullness:

• 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.

Final

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

Extra:

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)

Some simple rules to remember -

Rules:

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)

Extras:

Constexpr Constructor Rules:

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

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

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.

Altough it is more thoughly explained in the "Constexpr - Generalized Constant Expressions in C++11", Alex Allain Article, I will simplify it more with a couple simple points.

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

Simple explanation of Usefullness:

• 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.

Final

• 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

Extra:

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)

Some simple rules to remember -

Rules:

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)

Extras:

Constexpr Constructor Rules:

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)

Citation:

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

Altough it is more thoughly explained in the "Constexpr - Generalized Constant Expressions in C++11", Alex Allain Article, I will simplify it more with a couple simple points.

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

Simple explanation of Usefullness:

• 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.

Final

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

Extra:

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)

Some simple rules to remember -

Rules:

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)

Extras:

Constexpr Constructor Rules:

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

Altough it is more thoughly explained in the "Constexpr - Generalized Constant Expressions in C++11", Alex Allain Article, I will simplify it more with a couple simple points.

//EDIT

  Sorry, It was my fault to make it seem like I was the author of these points and I have corrected myself and changed various parts and add citations to avoid plagarism.

Simple explanation of Usefullness:

• 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.

Final

• 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

Extra:

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)

Some simple rules to remember -

Rules:

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)

Extras:

Constexpr Constructor Rules:

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)

Citation:

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

Altough it is more thoughly explained in the "Constexpr - Generalized Constant Expressions in C++11", Alex Allain Article, I will simplify it more with a couple simple points.

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

Simple explanation of Usefullness:

• 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.

Final

• 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

Extra:

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)

Some simple rules to remember -

Rules:

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)

Extras:

Constexpr Constructor Rules:

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)

Citation:

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