[英]Visual C++ Linking LNK2019 Error with a Precompiled Header
I had a very weird problem with precompile header. 我对预编译标头有一个很奇怪的问题。 The linker generates LNK2019: unresolved external symbol error when I implement method in .cpp file.
当我在.cpp文件中实现方法时,链接器生成LNK2019:未解决的外部符号错误。 However, the program could be compiled if I implement method in .h file.
但是,如果我在.h文件中实现方法,则可以编译该程序。 I happened to find out a solution but I have no idea about the root cause of this error.
我碰巧找到了解决方案,但是我不知道此错误的根本原因。
My project structure looks like this 我的项目结构如下所示
Solution 解
->project 1 ->项目1
->project 2 ->项目2
Project 1 has 3 files. 项目1有3个文件。 Ah, A.cpp and stdafx.h
啊,A.cpp和stdafx.h
file A.h
#pragma once
class A
{
public:
int num;
A();
};
file A.cpp
#include "stdafx.h"
A::A()
{
num = 2;
}
file stdafx.h
...
#include "A.h"
...
In project 2. I want to use A class. 在项目2中。我想使用A类。
file whatever.cpp 文件every.cpp
#include "stdafx.h"
#include "../Project1/A.h"
...
A* a_obj = new A();
...
In compile time, the linker reports unresolved external symbol error for the A construction function. 在编译时,链接器报告A构造函数的未解决的外部符号错误。 If I implement the constructor in Ah file.
如果我在Ah文件中实现构造函数。 the project2 could be successful complied.
project2可能会成功完成。 I want to know, why can not put the implementation in A.cpp file?
我想知道,为什么不能将实现放在A.cpp文件中? What is the right way to organize precompile header?
组织预编译头的正确方法是什么?
Thank you 谢谢
Project 2 does not include the definition of the A constructor - one way to give it visibility of this is to include the definition in the header file (which you did). 项目2不包括A构造函数的定义-一种使它具有可见性的方法是将定义包括在头文件中(您已这样做)。
Another way would be to include the A.cpp file in project 2. 另一种方法是在项目2中包含A.cpp文件。
A third way would be to export the A class, or the A constructor using a .def file or using the dllexport
directive. 第三种方法是使用.def文件或
dllexport
指令导出A类或A构造函数。
Put this in the precompiled header file: 将其放入预编译的头文件中:
// set up export import macros for client project use
// define the symbol "PROJ1" before including this file in project 1 only
// leave it undefined for other projects
#ifdef PROJ1
#define DLLEXP __declspec(dllexport)
#else
#define DLLEXP __declspec(dllimport)
#endif
Then declare the A class in the A header: 然后在A标头中声明A类:
DLLEXP class A
{
public:
A();
...
};
Or: 要么:
class A
{
public:
DLLEXP A();
...
};
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