In C++, you can declare a function and define its implementation separately. Here's the syntax for declaring and defining a function:

// function declaration (prototype)
return_type function_name(parameter_list);

// function definition
return_type function_name(parameter_list) {
    // function body
    // ...
    return result;
}

The return_type is the data type that the function returns. The function_name is the name of the function. The parameter_list is a comma-separated list of parameters that the function takes, with each parameter consisting of a data type and a name.

For example, here's a declaration and definition of a function that takes two integer parameters and returns their sum:

// function declaration (prototype)
int sum(int a, int b);

// function definition
int sum(int a, int b) {
    int result = a + b;
    return result;
}

In this example, the sum function is declared with the int return type, the name sum, and two int parameters a and b. The function definition then provides the implementation of the function, which simply adds the two parameters together and returns the result.

Note that the function declaration (prototype) is not required if the function definition appears before the code that calls the function. In that case, the compiler will infer the function's signature based on its definition. However, it is good practice to declare functions before using them to avoid compilation errors.

Here's an example of how to call the sum function:

int main() {
    int a = 2;
    int b = 3;
    int result = sum(a, b);
    std::cout << "The sum of " << a << " and " << b << " is " << result << std::endl;
    return 0;
}

This calls the sum function with a and b as arguments, and stores the result in the result variable. The program then outputs the result to the console.