What impact does the order of declaration and definition of C++ functions have?

In C, the order of function declaration and definition affects the compilation and linking process. The most common is declaration first and definition after; you can also use "forward declaration" to place the definition before the declaration; if both exist at the same time, the compiler will ignore the declaration and only use the definition.

The impact of the order of function declaration and definition

In C, both the declaration and definition of a function must appear in the program , the order between them will affect the compilation and linking process of the code.

Declaration

The function declaration informs the compiler of the existence of the function, including the function name, parameter type and return value type. Its syntax is as follows:

returnType functionName(parameterTypes);

For example:

int add(int, int);

Definition

The function definition provides the implementation of the function, including the code body. Its syntax is as follows:

returnType functionName(parameterTypes) {
  // 函数体
}

Order effect

• Declarations come first, definitions follow: This is the most common order. The compiler sees the function declaration at compile time and matches it with the function definition at link time.
• Definition first, declaration last: This order is called "forward declaration". The compiler finds function definitions through function declarations at link time. This order is typically used when placing function definitions in separate files or when resolving circular dependencies.
• Both at the same time: If both the declaration and definition of a function appear in the same location, the compiler ignores the declaration and treats the definition as the function's only definition.

Practical case

The following code demonstrates the order of function declaration first and definition last:

// main.cpp
// 函数声明在前

int add(int, int); // 声明函数

int main() {
  int result = add(10, 20); // 调用函数
  return 0;
}

// other_file.cpp
// 函数定义在后

int add(int a, int b) { // 定义函数
  return a + b;
}

In this order, compile The compiler will see the function declaration in main.cpp and match it during the linking phase with the function definition in other_file.cpp.

Conclusion

Understanding the order of function declarations and definitions in C is crucial because it affects the compilation and linking process. There is flexibility to use different sequences to structure the code as needed.

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