Why Does Template-argument Deduction Work Differently For Function-templates Compared To Class-templates (via Their Constructors)?

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Introduction

Template argument deduction (TAD) is a powerful feature in C++ that allows the compiler to automatically deduce the template arguments for a function or class template. However, when it comes to function templates and class templates, TAD works differently. In this article, we will explore why template-argument deduction works differently for function-templates compared to class-templates (via their constructors).

Template Argument Deduction (TAD)

Template argument deduction is a process that allows the compiler to automatically deduce the template arguments for a function or class template. The compiler uses a set of rules to deduce the template arguments based on the types of the function or class parameters. TAD is a crucial feature in C++ that simplifies the process of writing generic code.

Function Templates and TAD

Function templates are templates that define a function with template parameters. When a function template is instantiated, the compiler deduces the template arguments based on the types of the function parameters. The compiler uses a set of rules to deduce the template arguments, including:

  • Exact match: If the function parameter matches the template parameter exactly, the compiler deduces the template argument.
  • Conversion: If the function parameter can be converted to the template parameter, the compiler deduces the template argument.
  • Default argument: If the function parameter has a default argument, the compiler deduces the template argument.

Class Templates and TAD

Class templates are templates that define a class with template parameters. When a class template is instantiated, the compiler deduces the template arguments based on the types of the class parameters. However, class templates work differently than function templates when it comes to TAD. The compiler uses a set of rules to deduce the template arguments, including:

  • Constructor: If the class has a constructor that takes a single argument, the compiler deduces the template argument based on the type of the constructor argument.
  • Conversion: If the class has a conversion operator that takes a single argument, the compiler deduces the template argument based on the type of the conversion operator argument.
  • Default argument: If the class has a default argument for a constructor or conversion operator, the compiler deduces the template argument.

Why does TAD work differently for function-templates and class-templates?

The reason why TAD works differently for function-templates and class-templates is due to the way the compiler deduces the template arguments. Function templates are instantiated based on the types of the function parameters, whereas class templates are instantiated based on the types of the class parameters. When it comes to class templates, the compiler uses the constructor or conversion operator to deduce the template arguments, whereas function templates use the function parameters.

Example: FixedStr Class Template

Let's consider an example of a class template called FixedStr that is implicitly constructible from a string-literal:

template<FixedStr K, class... T>
struct ParserCtx {
  Parser<T...> parser;

  // ...
};

In this example, the FixedStr class template is instantiated with a single template argument K and a parameter pack T. The ParserCtx class template has a constructor that takes a single argument of type Parser<T...>.

Example: TAD with FixedStr Class Template

Let's consider an example of TAD with the FixedStr class template:

ParserCtx<std::string, int> ctx("hello", 42);

In this example, the ParserCtx class template is instantiated with the std::string type as the template argument K and the int type as the template argument T. The compiler deduces the template arguments based on the types of the constructor arguments.

Conclusion

In conclusion, template-argument deduction works differently for function-templates compared to class-templates (via their constructors). The reason for this difference is due to the way the compiler deduces the template arguments. Function templates are instantiated based on the types of the function parameters, whereas class templates are instantiated based on the types of the class parameters. When it comes to class templates, the compiler uses the constructor or conversion operator to deduce the template arguments, whereas function templates use the function parameters.

References

Further Reading

Introduction

Template argument deduction (TAD) is a powerful feature in C++ that allows the compiler to automatically deduce the template arguments for a function or class template. However, TAD can be complex and confusing, especially for beginners. In this article, we will answer some frequently asked questions about TAD in C++.

Q1: What is Template Argument Deduction (TAD)?

A1: Template argument deduction is a process that allows the compiler to automatically deduce the template arguments for a function or class template. The compiler uses a set of rules to deduce the template arguments based on the types of the function or class parameters.

Q2: How does TAD work for function templates?

A2: TAD works for function templates by using a set of rules to deduce the template arguments based on the types of the function parameters. The compiler uses exact match, conversion, and default argument rules to deduce the template arguments.

Q3: How does TAD work for class templates?

A3: TAD works for class templates by using a set of rules to deduce the template arguments based on the types of the class parameters. The compiler uses constructor, conversion, and default argument rules to deduce the template arguments.

Q4: What is the difference between TAD for function templates and class templates?

A4: The main difference between TAD for function templates and class templates is that function templates are instantiated based on the types of the function parameters, whereas class templates are instantiated based on the types of the class parameters.

Q5: Can I use TAD with non-type template parameters?

A5: Yes, you can use TAD with non-type template parameters. However, the compiler will only deduce the template arguments if the non-type template parameter is a constant expression.

Q6: Can I use TAD with template template parameters?

A6: Yes, you can use TAD with template template parameters. However, the compiler will only deduce the template arguments if the template template parameter is a template template parameter.

Q7: What are the limitations of TAD?

A7: The main limitations of TAD are that it can only deduce template arguments based on the types of the function or class parameters, and it cannot deduce template arguments based on the values of the function or class parameters.

Q8: Can I use TAD with C++11/C++14/C++17/C++20?

A8: Yes, you can use TAD with C++11/C++14/C++17/C++20. However, the syntax and rules for TAD have changed slightly between each version of the standard.

Q9: Can I use TAD with non-C++ standard libraries?

A9: Yes, you can use TAD with non-C++ standard libraries. However, the library must provide a way for the compiler to deduce the template arguments.

Q10: Can I use TAD with my own custom libraries?

A10: Yes, you can use TAD with your own custom libraries. However, you must provide a way for the compiler to deduce the template arguments.

Conclusion

In conclusion, template argument deduction is a powerful feature in C++ that allows the compiler to automatically deduce the template arguments for a function or class template. However, TAD can be complex and confusing, especially for beginners. By understanding the rules and limitations of TAD, you can write more efficient and effective code.

References

Further Reading