您需要在main之前提到重载calcPostage函数的函数原型，这将解决问题.

You need to mention the function prototype for overloaded calcPostage functions before main,that will solve the issue.

编译器从源文件的顶部到底部开始工作.

函数原型是对将要存在的函数的定义，因此编译器知道不会使代码编译失败.

如果发现尚未定义的功能，它将失败，以同样的方式，如果定义了两次，则将失败.

正如Harish所说的那样:

int calcPostage(int);
int calcPostage(int，int，int);

在main将要修复代码之前，以便编译器知道这些功能将在代码的将来存在.

The compiler works from the top of a source file to the bottom.

A function prototype is a definition of the function that will exist and so the compiler knows not to fail the code compile.

If it finds a function that is not defined already it will fail, in the same way it will fail if a function is defined twice.

As Harish has said putting:

int calcPostage(int);
int calcPostage(int,int,int);

before the main will fix the code so that the compiler knows that these function will exist at some point in the future in the code.