Which implementation of C++ templates is better?

Comparison of explicit instantiation and implicit instantiation: Explicit instantiation allows finer control over code generation, avoiding errors and speeding up compilation. Implicit instantiation is more convenient, general, and avoids duplication, but may take longer to compile and may cause code bloat. Recommended use: Use implicit instantiation in most cases, but explicit instantiation may be more appropriate for specific cases where optimization is required, disabling implicit instantiation, or reducing compile time/code size.

Explicit and Implicit instantiation of C++ templates: Which is better?

In C++ template programming, there are two ways to instantiate templates: explicit instantiation and implicit instantiation. Both have their pros and cons, and understanding their differences can help make the right choice.

Explicit Instantiation

explicit Instantiation explicitly creates a specific template instance. The syntax is as follows:

template<>
class MyClass<T> {
  ...
};

Implicit instantiation

When the compiler uses a template, if an explicit instantiation does not exist, the compiler will automatically generate an implicit instantiation.

Advantages

• Code generation control: Explicit instantiation provides fine-grained control over code generation, allowing optimization of specific instances.
• Avoid compilation errors: For templates with complex template parameters, explicit instantiation can prevent compilation errors because the compiler does not try to generate all possible instances.
• Faster compilation: For rarely used templates, explicit instantiation can reduce compilation time because the compiler only generates instances that are actually needed.

Disadvantages

• Length: For large or general templates, writing explicit instantiations can be verbose.
• Error-prone: If you forget to instantiate a specific template instance, it may cause compilation errors.

Implicit instantiation

Advantages

• Convenient: Not required Manually write instantiation code.
• Universal: Automatically applies to all template parameter values.
• Avoid duplication: The compiler only generates template instances once.

Disadvantages

• Long compilation time: For large or general templates, the compiler generates all possible instances Will result in longer compilation time.
• Code bloat: May cause the executable size to increase due to all instances being generated.

Practical case

Consider a template function that calculates the maximum of two numbers:

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

explicit Instantiation:

// 显式实例化整数版本
template<>
inline int max<int>(int a, int b) {
  return (a > b) ? a : b;
}

Implicit Instantiation:

No need for explicit instantiation, the compiler will automatically generate instances of all types when used.

Recommended use

In most cases, implicit instantiation is a more convenient and versatile method. However, explicit instantiation may be superior for

when
• when optimization is required for a specific type of template instance.
• When implicit instantiation of a specific type is explicitly prohibited.

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