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In C++11, this technique is known as generalized constant expressions (constexpr). [2] C++14 relaxes the constraints on constexpr – allowing local declarations and use of conditionals and loops (the general restriction that all data required for the execution be available at compile-time remains).
Expression templates are a C++ template metaprogramming technique that builds structures representing a computation at compile time, where expressions are evaluated only as needed to produce efficient code for the entire computation. [1]
C++26 is the informal name for the version of the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC) 14882 standard for the C++ programming language that follows C++23. The current working draft of this version is N4981.
A third way is by declaring and defining a variable as being "constant". A global variable or static variable can be declared (or a symbol defined in assembly) with a keyword qualifier such as const, constant, or final, meaning that its value will be set at compile time and should not be changeable at runtime. Compilers generally put static ...
Both expressions have the same meaning and behave in exactly the same way. The latter form was introduced to avoid confusion, [3] since a type parameter need not be a class until C++20. (It can be a basic type such as int or double.) For example, the C++ Standard Library contains the function template max(x, y) which returns the larger of x and ...
However, C++11 constexpr functions could only contain a single expression that is returned (as well as static_asserts and a small number of other declarations). C++14 relaxes these restrictions. Constexpr-declared functions may now contain the following: [3] Any declarations except: static or thread_local variables.
because the argument to f must be a variable integer, but i is a constant integer. This matching is a form of program correctness, and is known as const-correctness.This allows a form of programming by contract, where functions specify as part of their type signature whether they modify their arguments or not, and whether their return value is modifiable or not.
Here, attempting to use a non-class type in a qualified name (T::foo) results in a deduction failure for f<int> because int has no nested type named foo, but the program is well-formed because a valid function remains in the set of candidate functions.