How does C++ define the concept for template parameters
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T.10: Define concepts for all template parameters
Reason
Correctness and readability. The assumed meaning (syntax and semantics) of a template argument is fundamental to the interface of a template. A concept dramatically improves documentation and error handling for the template. Specifying concepts for template arguments is a powerful design tool.
correctness and readability. The assumed meaning (syntax and semantics) of a template parameter is the basis of the template interface. The concept greatly improves template documentation and error handling. Defining concepts for template parameters is a powerful design tool.
Example
template
// requires Input_iterator
// && Equality_comparable
Iter find(Iter b, Iter e, Val v)
{
// ...
}
or equivalently and more succinctly:
Or use the following functionally equivalent, but simpler, approach:
template
// requires Equality_comparable
Iter find(Iter b, Iter e, Val v)
{
// ...
}Note
"Concepts" are defined in an ISO Technical Specification: concepts. A draft of a set of standard-library concepts can be found in another ISO TS: ranges Concepts are supported in GCC 6.1 and later. Consequently, we comment out uses of concepts in examples; that is, we use them as formalized comments only. If you use GCC 6.1 or later, you can uncomment them:
"Concept" is defined by ISO technical specifications: concepts. A preliminary version of a standard library concept can be found in another ISO specification: ranges. GCC 6.1 supports concepts. So we commented out concepts in our example code; that is, we used them as standard comments. If you are using a later version of GCC 6.1, you can open comments.
template
requires Input_iterator
&& Equality_comparable
Iter find(Iter b, Iter e, Val v)
{
// ...
}Note
Plain typename (or auto) is the least constraining concept. It should be used only rarely when nothing more than "it's a type" can be assumed. This is typically only needed when (as part of template metaprogramming code) we manipulate pure expression trees, postponing type checking.
A straightforward type name (or auto) is the concept of minimal constraint. It should be used sparingly and limited to expressing "it is a type." This is usually only necessary (as part of template metaprogramming) when we operate on pure expression trees, deferred type checking.
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