Class templates are common classes in C that can handle different types of data, avoiding duplicate code. It is defined through the template keyword, and uses typename or class to specify type parameters, such as template class Box { T value; }; when used, specify a specific type such as Box, and the compiler will automatically generate the corresponding class based on the type. Class templates are only instantiated when used, so the code needs to be placed in the header file. The template can have multiple parameters, default parameters, specialized processing of specific types, or accept non-type parameters such as integers. It is recommended to master the basic structure when you are beginners, refer to the STL source code, and test in segments during debugging.

When writing C code, if you want a class to handle different types of data but don’t want to repeat many similar codes, then Class Templates are your good helper. It is not magic, nor is it particularly difficult. It is just that many people think it is circumcised at first, but it is quite natural after understanding the structure.

What is a class template?

Simply put, class templates are a way to make classes support generic programming. You can understand it as a "mold for making cakes". The mold itself does not have a specific taste. It will only become a specific class when you pour the material (such as int, float or custom type) into it.

For example, std::vector in the standard library is a class template that can install int, string, and even classes you write yourself without having to write a vector for each type.

How to define a class template?

The keyword that defines a class template is template , followed by <typename t></typename> or <class t></class> (both are basically the same), and then write the class structure normally.

For example:

 template <typename T>
class Box {
private:
    T value;
public:
    Box(T v) : value(v) {}
    void print() { std::cout << value << std::endl; }
};

This code defines a box that can hold any type of data. When used it like this:

 Box<int> intBox(123);
Box<std::string> strBox("Hello");

The compiler will automatically generate the corresponding class for you based on the type you pass in.

What is the difference between a class template and a normal class?

The biggest difference is that the class template does not generate code directly at compile time and will only be instantiated when it is used . That is, if you only write the template but don't use it, the compiler won't generate any actual class code.

This has several implications:

• If you write a complex class template but use only a small part of the functionality, the other unused parts may not be compiled at all.
• The template code must be placed in the header file, and the declaration and implementation cannot be separated like ordinary classes. Because the compiler needs to see the complete template definition in order to instantiate it.

What should you pay attention to when using class templates?

1. There can be multiple template parameters

    template <typename T, typename U>
   class Pair {
       T first;
       U second;
   };

2. Default template parameters

    template <typename T = int>
   class MyContainer {};

   In this way, if no type is specified, int will be used by default.

3. Specialization is important Sometimes you want to do different processing for certain types, such as copying strings to char* instead of pointer assignments, so you need specialization:

    template <>
   class Box<char*> {
       // Special processing logic};

4. Template parameters are not just types You can also pass non-type parameters such as integers:

    template <int Size>
   class StaticArray {
       int data[Size];
   };

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   Some tips for writing templates

   • Don’t pursue complexity when you are beginners, and first master the basic structure.
   • If you look at the source code of STL, such as the template implementation of vector and map, you can learn a lot of routines.
   • When debugging template errors, don't be afraid to see the compiler errors. Although there is a lot of information, the prompt position is usually very accurate.
   • If the template error occurs, try to simplify the problem and test it in segments, and do not pile too many functions at once.

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   Basically that's it. Class templates look a bit abstract, but if you use them too much, you will find that it is actually a tool that allows you to write less repetitive code, which is not complicated but is easy to ignore details. Take your time and experience it while writing.

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