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This article introduces the knowledge of "what are the different implementation methods of C++ using tags to provide functions?". In the operation of actual cases, many people will encounter such a dilemma. Next, let the editor lead you to learn how to deal with these situations! I hope you can read it carefully and be able to achieve something!

T.65: use label distribution to provide different implementations of functions

Reason (reason)

A template defines a general interface.

The template defines the universal interface.

Tag dispatch allows us to select implementations based on specific properties of an argument type.

Label distribution allows us to choose the implementation based on the specific properties of the parameter type.

Performance.

Performance

Example (sample)

This is a simplified version of std::copy (ignoring the possibility of non-contiguous sequences)

This is a simplified version of std::copy (ignoring discontiguous sequences)

Struct pod_tag {}

Struct non_pod_tag {}

Template struct copy_trait {using tag = non_pod_tag;}; / / T is not "plain old data"

Template struct copy_trait {using tag = pod_tag;}; / / int is "plain old data"

Template

Out copy_helper (Iter first, Iter last, Iter out, pod_tag)

{

/ / use memmove

}

Template

Out copy_helper (Iter first, Iter last, Iter out, non_pod_tag)

{

/ / use loop calling copy constructors

}

Template

Out copy (Iter first, Iter last, Iter out)

{

Return copy_helper (first, last, out, typename copy_trait::tag {})

}

Void use (vector& vi, vector& vi2, vector& vs, vector& vs2)

{

Copy (vi.begin (), vi.end (), vi2.begin ()); / / uses memmove

Copy (vs.begin (), vs.end (), vs2.begin ()); / / uses a loop calling copy constructors

}

This is a general and powerful technique for compile-time algorithm selection.

This is a universal and powerful technique that allows you to select algorithms at compile time.

Note (Note)

When concepts become widely available such alternatives can be distinguished directly:

When concepts can be widely used, such options can directly distinguish between:

Template

Requires Pod

Out copy_helper (In, first, In last, Out out)

{

/ / use memmove

}

Template

Out copy_helper (In, first, In last, Out out)

{

/ / use loop calling copy constructors

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