C++ 变量有初始值设定项但类型不完整？
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C++ variable has initializer but incomplete type?
提问by
I am trying to compile 2 classes in C++ with the following command:
我正在尝试使用以下命令在 C++ 中编译 2 个类：
g++ Cat.cpp Cat_main.cpp -o Cat
g++ Cat.cpp Cat_main.cpp -o Cat
But I receive the following error:
但我收到以下错误：
Cat.cpp:10:10: error: variable ‘Cat Joey' has initializer but incomplete type
Cat.cpp:10:10: error: variable ‘Cat Joey' has initializer but incomplete type
Could someone explain to me what this means? What my files basically do is create a class (Cat.cpp) and create an instance (Cat_main.cpp). Here is my source code:
有人可以向我解释这是什么意思吗？我的文件基本上做的是创建一个类 ( Cat.cpp) 并创建一个实例 ( Cat_main.cpp)。这是我的源代码：
Cat.cpp:
猫.cpp：
#include <iostream>
#include <string>
class Cat;
using namespace std;
int main()
{
Cat Joey("Joey");
Joey.Meow();
return 0;
}
Cat_main.cpp:
猫_main.cpp：
#include <iostream>
#include <string>
using namespace std;
class Cat
{
public:
Cat(string str);
// Variables
string name;
// Functions
void Meow();
};
Cat::Cat(string str)
{
this->name = str;
}
void Cat::Meow()
{
cout << "Meow!" << endl;
return;
}
采纳答案by Luchian Grigore
You use a forward declaration when you need a complete type.
当您需要完整类型时，您可以使用前向声明。
You must have a full definition of the class in order to use it.
您必须拥有该类的完整定义才能使用它。
The usual way to go about this is:
通常的方法是：
1) create a file Cat_main.h
1）创建文件 Cat_main.h
2) move
2）移动
#include <string>
class Cat
{
public:
Cat(std::string str);
// Variables
std::string name;
// Functions
void Meow();
};
to Cat_main.h. Note that inside the header I removed using namespace std;and qualified string with std::string.
到Cat_main.h。请注意，在标题中，我删除了using namespace std;字符串并使用std::string.
3) include this file in both Cat_main.cppand Cat.cpp:
3) 在Cat_main.cpp和中都包含此文件Cat.cpp：
#include "Cat_main.h"
回答by Dmitriy Sukharev
It's not related to Ken's case directly, but such an error also can occur if you copied .hfile and forgot to change #ifndefdirective. In this case compiler will just skip definition of the class thinking that it's a duplication.
它与 Ken 的案例没有直接关系，但是如果您复制了.h文件并且忘记更改#ifndef指令，也会发生这样的错误。在这种情况下，编译器将跳过类的定义，认为它是重复的。
回答by David Rodríguez - dribeas
You cannot define a variable of an incomplete type. You need to bring the whole definition of Catinto scope beforeyou can create the local variable in main. I recommend that you move the definition of the type Catto a header and include it from the translation unit that has main.
您不能定义不完整类型的变量。您需要将 的整个定义Cat引入范围，然后才能在main. 我建议您将类型的定义移动Cat到标题中，并将其从具有main.
回答by xtluo
Sometimes, the same error occurs when you forget to include the corresponding header.
有时，当您使用forget to include the corresponding header.
回答by peter karasev
I got a similar error and hit this page while searching the solution.
我在搜索解决方案时遇到了类似的错误并点击了此页面。
With Qt this error can happen if you forget to add the QT_WRAP_CPP( ... )step in your build to run meta object compiler (moc). Including the Qt header is not sufficient.
对于 Qt，如果您忘记QT_WRAP_CPP( ... )在构建中添加步骤以运行元对象编译器 (moc)，则可能会发生此错误。包括 Qt 头文件是不够的。
