Type deduction mechanism of C++ function templates

Function templates infer return types and types at compile time through the type inference mechanism, allowing the creation of general functions or classes with different types of parameters. Type derivation rules include: perfect forwarding: template parameters are passed directly from the parameter type in the function prototype; template parameter inference: the compiler infers the type of template parameters from the parameter type, starting from the most specific parameter type.

#C Type derivation mechanism of function template

What is a function template? How can I accomplish type deduction for a function template during compilation so that the compiler can automatically infer the return type and other types based on the parameter types when instantiating the template function?

Template mechanism

A template is a programming Construct that allows the creation of a common set of functions or classes that can be used for multiple data types. By using appropriate syntax, we can use type parameters while writing a function or class and then call the template with different types of parameters.

Type derivation

Type derivation refers to the process of automatically inferring the return type or other types from the function parameter types. In function templates, unknown types are specified using template parameters, and the compiler infers these unknown types by analyzing the parameter types in the template call.

Deduction rules

Type derivation follows the following rules:

1. Perfect forwarding: If the template parameter appears in the function parameter type of the function prototype , its type will be passed directly to the corresponding parameter type of the called function.

2. Template parameter inference: If a template parameter appears in the return type or other type of a function prototype, the compiler will try to infer its type from the function parameter type. It uses the following steps:

   • Start with the most specific parameter type and look for types that match the template parameters.
   • If a match is found, the type of the template parameter is inferred.
   • If no match is found, the compiler will report an error.

Practical case

Consider the following function template:

template <typename T>
T sum(T a, T b) {
    return a + b;
}

When we call this template function, the compiler will infer based on the parameter type The type of T. For example:

int x = sum(10, 20);  // T 被推断为 int

In this example, T is inferred as int because both arguments are of type int. Therefore, the function returns an int.

Notes

When performing type derivation, there are several points to note:

• The derived type must match the constraints of the template parameters.
• Cannot infer type from empty parameter list.
• When template parameters appear in return types and parameter types, the derivation rules become more complex.

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