Static Library C++ Unresolved External Symbol

a_unique_name 401

Hello Folks:

Developing on Win 10 Pro, VS Community 2022 updated today, all C++.

This feels like I must be doing something stupid. Perhaps I forgot to do something.

I like to use static libraries.

I created a new library called common_diagnostic to give all of my diagnostic output a common look and feel.

I've started with the library having two files, header and source, that should define a const char * for use by other modules.

These files are just minimal to get a build to work:

#pragma once

#include "pch.h"

Building Application

I altered common_diagnostic.cpp to force it to be compiled with Build > Build Solution. It seems to compile without error. DEFAULT_DIAGNOSTIC_INDENT is defined in that file as a global variable.

Commctrl_diagnostics is a different static library, currently the only module that makes an external referenc to DEFAULT_DIAGNOSTIC_INDENT.

Here is the build report:

1>------ Build started: Project: common_diagnostic, Configuration: Debug Win32 ------

I think I've done what's needed for dependencies:

screenshot_1

screenshot_5

I've noticed the .cpp files in all of the other libraries I've built and used have #include "StdAfx.h". This library uses #include "pch.h".

I have no idea if that bit if information helps.

What did I forget?

Thanks
Larry

RLWA32 45,571 Reputation points

2024-09-20T07:30:01.09+00:00

Stdafx.h is an older default name for the precompiled header. C++ projects created by newer versions of VS use pch.h. Typically you would not declare/define anything in these headers and only use them to contain #include statements. Maybe one of these is missing a needed #include for a header that defines the missing variable.

Unresolved external symbol seems like a linker error. Maybe the project that uses your static libraries is not linking to the library that defines the variable associated with that symbol. Or maybe that library has not made that symbol visible.

But you really haven't given us much information to go on. Several of the items in your question seem to be incomplete (e.g., build report and code).

RLWA32 45,571

A simple example -

StatLib1.cpp

// StatLib1.cpp : Defines the functions for the static library.
//

#include "pch.h"
#include "framework.h"
#include "stdio.h"

const char* GLOBAL_VARIABLE;

// TODO: This is an example of a library function
void fnStatLib1()
{
    GLOBAL_VARIABLE = "GlobalText";
    printf_s("StatLib 1 set variable to %s\n", GLOBAL_VARIABLE);
}

StatLib1.h

#pragma once
extern const char* GLOBAL_VARIABLE;

StatLib2.cpp

// StatLib2.cpp : Defines the functions for the static library.
//

#include "pch.h"
#include "framework.h"
#include <stdio.h>
#include "..\StatLib1\StatLib1.h"

// TODO: This is an example of a library function
void fnStatLib2()
{
    printf("Statlib 2 use global variable %s\n", GLOBAL_VARIABLE);
}

Basic19.cpp

#include <stdio.h>
#include <tchar.h>

#pragma comment(lib, "StatLib1")
#pragma comment(lib, "StatLib2")

void fnStatLib1();
void fnStatLib2();


int main()
{
    fnStatLib1();
    fnStatLib2();

    return 0;
}

All is well -

enter image description here

Now, make the GLOBAL_VARIABLE static in StatLib1.cpp

Oops, its not visible.

enter image description here

Remove static and make the following change to Basic19.cpp

#include <stdio.h>
#include <tchar.h>

//#pragma comment(lib, "StatLib1")
#pragma comment(lib, "StatLib2")

//void fnStatLib1();
void fnStatLib2();


int main()
{
    //fnStatLib1();
    fnStatLib2();

    return 0;
}

Now since we're not linking with StatLib1.lib -

enter image description here

Darran Rowe 916 Reputation points

2024-09-22T11:11:03.3733333+00:00

There are two small notes about the precompiled header, but the mechanism that you see on the surface is pretty simple.

First, the file name doesn't matter. The name implies that it came about due to MFC (stdafx.h), and considering how heavy the MFC headers were back in the day, it makes sense. But if the precompiled header is used for including WinRT/WinUI headers into a WinUI 3 application, then the name is meaningless, so a simpler pch.h makes sense.

Secondly, the name is nothing more than an indicator to the compiler to include the parse tree at a certain point in the file.

There are three compiler options that are used for the generation and use of precompiled headers. First is /Fp, this option names the .pch file. The .pch file is the file which contains a binary representation of the headers included in the precompiled headers.

Then there is /Yc, this option is used on the stdafx.cpp or pch.cpp file. This tells the compiler that this source file is going to be used to generate the .pch file. There are two variants of /Yc, if /Yc is used on its own, then the entire file is used in the .pch file. If a file name is specified with /Yc then everything up to and including the file is included in the .pch file. There is no requirement on the name of the file that /Yc is used on, and there is no requirement on the name of the file used as part of the /Yc compiler option.

Finally, there is /Yu. This compiler option takes a file name, and the compiler simply finds the #include statement that includes the file named by /Yu and just uses the file named by /Fp to generate the initial state of the compilation.

There are a few extra options for controlling precompiled headers, like the hdrstop pragma directive. You really could name the precompiled header file moomoo.h and the precompiled output as baabaa.pch and the compiler wouldn't care.
Minxin Yu 11,751 Reputation points Microsoft Vendor

2024-09-23T02:29:06.6433333+00:00

Is there any update?
RLWA32 45,571 Reputation points

2024-09-24T15:05:18.51+00:00

Larry, where do you stand with this issue?

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Static Library C++ Unresolved External Symbol

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