c – 错误：未在此范围内声明变量

当我尝试编译下面的代码时,我收到以下错误.

Error: main.cpp: In function "int main()":
       main.cpp:6: error: "display" was not declared in this scope

test1.h

#include<iostream.h>
class Test
{
  public:
    friend int display();
};

test1.cpp：

#include<iostream.h>
int  display()
{
    cout<<"Hello:In test.cc"<< endl;
    return 0;
}

main.cpp中

#include<iostream.h>
#include<test1.h>
int main()
{
 display();
 return 0;
}

奇怪的是我能够成功地在unix中编译.
我正在使用gcc和g编译器

解决方法:

在将其声明为朋友之前,您需要提供该函数的声明.
作为朋友的声明不符合标准的实际功能声明.

C 11标准§7.3.1.2[namespace.memdef]：
第3段：

[…] If a friend declaration in a nonlocal class first declares a class or function the friend class or function is a member of the innermost enclosing namespace. The name of the friend is not found by unqualified lookup or by qualified lookup until a matching declaration is provided in that namespace scope (either before or after the class definition granting friendship). […]

#include<iostream.h>
class Test
{
  public:
    friend int display();  <------------- Only a friend declaration not actual declaration
};

你需要：

#include<iostream.h>
int display();            <------- Actual declaration
class Test
{
  public:
    friend int display();     <------- Friend declaration 
};

标签：c,cbuilder
来源： https://codeday.me/bug/20190901/1780528.html