Partial specialization and explicit instantiation of C++ function templates

C function templates provide partial specialization and explicit instantiation to achieve custom implementations of special types. Partial specialization: Allows custom implementations to be provided for specific types, taking precedence over generic implementations. Explicit instantiation: Forces the creation of implementations of specific types at compile time, improving runtime efficiency.

C Partial specialization and explicit instantiation of function templates

In C, function templates can define a general Functions, which can be used on different types of data. In some cases, a different implementation of a specific type of function may be required. This can be achieved through partial specialization and explicit instantiation of the function template.

Partial specialization

Partial specialization allows to provide alternative implementations for specific type parameters of function templates. The syntax is as follows:

template <typename T>
void my_function(T a, T b);

template <>
void my_function(int a, int b) {
  // 特定的实现
}

In this example, the my_function function is partially specialized to handle parameters of type int. When it is called, the int type implementation will be used instead of the generic implementation.

Explicit instantiation

Explicit instantiation forces a specific implementation of a function template to be created at compile time. The syntax is as follows:

template class my_function<int>;

When this instantiation is placed in a compilation unit, the int type version of the my_function function will be implemented immediately, rather than in the first when called once. This improves runtime efficiency but increases compilation time.

Practical case

Consider a max function that calculates the maximum value of two numbers. The generic implementation is as follows:

template <typename T>
T max(T a, T b) {
  return (a > b) ? a : b;
}

However, for the int type we can provide a faster implementation that uses assembly instructions to compare the registers directly:

template <>
int max(int a, int b) {
  int result;
  asm("movl %1, %%eax\n\tcmp %2, %%eax\n\tmovg %%eax, %0\n\tmovl %2, %%eax\n\tmovng %%eax, %0"
      : "=m"(result)
      : "g"(a), "g"(b));
  return result;
}

To use this specific implementation, we need to instantiate it explicitly:

template class max<int>;

Now, when the max function is called, it will use the specific int type implementation, Thereby improving its efficiency when processing int type data.

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