What are C 17 Template Deduction Guides and How Do They Work?

What are Template Deduction Guides in C 17?

C 17 introduced template deduction guides as a mechanism to derive template arguments automatically when instantiating template class objects. They facilitate the deduction of template parameters based on constructor arguments, addressing scenarios where traditional template argument deduction falls short.

Why and When to Use Template Deduction Guides?

Template deduction guides are necessary when the template parameter cannot be directly inferred from the type of any constructor argument. This typically occurs when the parameter type depends on traits or transformations applied to the argument type.

How to Declare Template Deduction Guides?

Template deduction guides are declared in the same fashion as constructors, except that they are followed by an arrow (->) and a template specialization. The left side of the arrow represents the constructor signature pattern that triggers the deduction, while the right side specifies the deduced template specialization:

template<typename iterator> void func(Iterator first, Iterator last)
{
  vector v(first, last);
}</typename>

In the above example, the vector template has a deduction guide that allows the compiler to deduce T based on the iterator traits:

template<typename iterator> vector(Iterator b, Iterator e) -> 
    vector<typename std::iterator_traits>::value_type>;</typename></typename>

Template deduction guides provide a convenient and concise way to handle complex template argument deduction, simplifying code and improving type safety.

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