const Vs constexpr

With C++11, WG21 comity introduced generalized constant expression to be able to use in C++ as compile time evaluated constants, and only be used on where its value is evaluated to a const expression. It should have the following requirements:
As a Variable:

• it must be immediately constructed or assigned a value.

• the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.

• the constructor used to construct the object (either implicit or explicit) must satisfy the requirements of constexpr constructor. In the case of explicit constructor, it must have constexpr specified.

As a Function:

• it must not be virtual
• its return type must be LiteralType
• each of its parameters must be literal type
• the function body must be either deleted or defaulted or contain only the following:

• null statements
• static_assert declarations
• typedef declarations and alias declarations that do not define classes or enumerations
• using declarations
• using directives
• exactly one return statement that contains only literal values, constexpr variables and functions.

As a Constructor:

• each of its parameters must be literal type
• the class must have no virtual base classes
• the constructor body must be either deleted or defaulted or contain only the following:

• null statements
• static_assert declarations
• typedef declarations and alias declarations that do not define classes or enumerations
• using declarations
• using directives

• the constructor must not have a function-try block
• every base class and every non-static member must be initialized, either in the initializer_list or by brace-or-equal initializer. In addition, every constructor involved must be a constexpr constructor and every clause of every brace-or-equal initializer must be a constant expression
• every implicit conversion involved must be a constant expression

See the below papaer for more information:

http://www.stroustrup.com/sac10-constexpr.pdf