c++ - 重载时未在 C++ 中的范围错误中声明

Question

我在尝试运行这段代码时遇到错误

In function 'int main()':
error: 'area' was not declared in this scope

我找不到问题的明确解决方案。

#include <iostream>

using namespace std;

int main() {

    area(13.3, 67.4);

    return 0;
}

void area(int a, int b){
    cout << "The area is " << a * b << endl;
}

void area(float a, float b){
    cout << "The area is " << a * b << endl;
}

void area(double a, double b){
    cout << "The area is " << a * b << endl;
}

Answer 1

您需要先声明这些函数，然后才能使用它们。

要么前向声明它们:

#include <iostream>

using namespace std;

// forward declarations
void area(int a, int b);
void area(float a, float b);
void area(double a, double b);

int main() {
    area(13.3, 67.4);
    return 0;
}

void area(int a, int b){
    cout << "The area is " << a * b << endl;
}

void area(float a, float b){
    cout << "The area is " << a * b << endl;
}

void area(double a, double b){
    cout << "The area is " << a * b << endl;
}

否则，将实现移至 main() 之上:

#include <iostream>

using namespace std;

void area(int a, int b){
    cout << "The area is " << a * b << endl;
}

void area(float a, float b){
    cout << "The area is " << a * b << endl;
}

void area(double a, double b){
    cout << "The area is " << a * b << endl;
}

int main() {
    area(13.3, 67.4);
    return 0;
}

话虽如此，由于实现完全相同，只是数据类型不同，请考虑改用单个模板函数:

#include <iostream>

using namespace std;

template<typename T>
void area(T a, T b){
    cout << "The area is " << a * b << endl;
}

int main() {
    area<double>(13.3, 67.4);
    return 0;
}