The C++17 standard introduces "template deduction guides". I gather they're something to do with the new template argument deduction for constructors introduced in this version of the standard, but I haven't yet seen a simple, FAQ-style explanation of what they are and what they're for.
What are template deduction guides in C++17?
Why (and when) do we need them?
How do I declare them?
Template argument deduction is used when selecting user-defined conversion function template arguments. A is the type that is required as the result of the conversion. P is the return type of the conversion function template.
As long as you are satisfied with automatic type inference, you can use a template constructor (of a non-template class). @updogliu: Absolutely.
Defining a Function TemplateA function template starts with the keyword template followed by template parameter(s) inside <> which is followed by the function definition. In the above code, T is a template argument that accepts different data types ( int , float , etc.), and typename is a keyword.
Template deduction guides are patterns associated with a template class that tell the compiler how to translate a set of constructor arguments (and their types) into template parameters for the class.
The simplest example is that of std::vector
and its constructor that takes an iterator pair.
template<typename Iterator> void func(Iterator first, Iterator last) { vector v(first, last); }
The compiler needs to figure out what vector<T>
's T
type will be. We know what the answer is; T
should be typename std::iterator_traits<Iterator>::value_type
. But how do we tell the compiler without having to type vector<typename std::iterator_traits<Iterator>::value_type>
?
You use a deduction guide:
template<typename Iterator> vector(Iterator b, Iterator e) -> vector<typename std::iterator_traits<Iterator>::value_type>;
This tells the compiler that, when you call a vector
constructor matching that pattern, it will deduce the vector
specialization using the code on the right of ->
.
You need guides when the deduction of the type from the arguments is not based on the type of one of those arguments. Initializing a vector
from an initializer_list
explicitly uses the vector
's T
, so it doesn't need a guide.
The left side doesn't necessarily specify an actual constructor. The way it works is that, if you use template constructor deduction on a type, it matches the arguments you pass against all deduction guides (actual constructors of the primary template provide implicit guides). If there is a match, it uses that to determine which template arguments to provide to the type.
But once that deduction is done, once the compiler figures out the template parameters for the type, initialization for the object of that type proceeds as if none of that happened. That is, the deduction guide selected does not have to match the constructor selected.
This also means that you can use guides with aggregates and aggregate initialization:
template<typename T> struct Thingy { T t; }; Thingy(const char *) -> Thingy<std::string>; Thingy thing{"A String"}; //thing.t is a `std::string`.
So deduction guides are only used to figure out the type being initialized. The actual process of initialization works exactly as it did before, once that determination has been made.
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