Resolving Unresolved External Symbol Errors in C++ Projects

In C++, an unresolved external symbol error occurs when the linker is unable to find the definition of a function or variable that is referenced in the code. This error is often encountered when working with multiple projects or libraries in a solution. In this tutorial, we will explore the causes of unresolved external symbol errors and discuss various methods for resolving them.

Understanding the Error

The unresolved external symbol error typically occurs during the linking phase of the compilation process. When the compiler encounters a reference to a function or variable, it checks if the definition is available in the current translation unit or in any of the included header files. If the definition is not found, the compiler generates an object file with a reference to the undefined symbol.

The linker then attempts to resolve these references by searching for definitions in other object files or libraries. If the linker is unable to find a definition for a referenced symbol, it reports an unresolved external symbol error.

Causes of Unresolved External Symbol Errors

There are several common causes of unresolved external symbol errors:

1. Missing Definitions: The most obvious cause is that the definition of the function or variable is missing.
2. Incorrect Link Order: If the object files or libraries are linked in the wrong order, the linker may not be able to find the definitions it needs.
3. Missing Library Dependencies: If a project depends on a library, but the library is not included in the link phase, the linker will report unresolved external symbol errors.

Resolving Unresolved External Symbol Errors

To resolve an unresolved external symbol error, you need to ensure that the definition of the referenced function or variable is available during the linking phase. Here are some methods for resolving these errors:

Method 1: Include the Definition File

One simple way to resolve an unresolved external symbol error is to include the definition file (e.g., .cpp file) in the project. This ensures that the compiler and linker have access to the definition during the compilation and linking phases.

However, including definition files directly can lead to multiple definition errors if the same file is included in multiple projects. A better approach is to create a separate library or object file that contains the definitions.

Method 2: Create a Static Library

A static library is a collection of object files that are linked together during the build process. To resolve an unresolved external symbol error, you can create a static library from the project that contains the definition and then link this library to the project that references the function or variable.

To create a static library in Visual Studio:

1. Open the project properties.
2. Navigate to the General tab under Configuration Properties.
3. Change the Configuration Type to Static Library (.lib).
4. Build the project to generate the static library file (e.g., MyProjectTest.lib).

Then, in the project that references the function or variable:

1. Open the project properties.
2. Navigate to the Linker tab under Configuration Properties.
3. Add the path to the static library directory under Additional Library Directories.
4. Add the name of the static library (e.g., MyProjectTest.lib) under Additional Dependencies.

Method 3: Create a Dynamic Library

A dynamic library is similar to a static library, but it is linked at runtime instead of during the build process. To create a dynamic library:

1. Open the project properties.
2. Navigate to the General tab under Configuration Properties.
3. Change the Configuration Type to Dynamic Library (.dll).
4. Build the project to generate the dynamic library file (e.g., MyProjectTest.dll).

Then, in the project that references the function or variable:

1. Open the project properties.
2. Navigate to the Linker tab under Configuration Properties.
3. Add the path to the dynamic library directory under Additional Library Directories.
4. Add the name of the import library (e.g., MyProjectTest.lib) under Additional Dependencies.

Method 4: Add a Project Reference

In some cases, adding a project reference between the projects can resolve an unresolved external symbol error. To add a project reference:

1. Right-click on the project in the solution explorer.
2. Click Add > References….
3. Check the boxes for the projects that this project relies on.
4. Click OK.

By following these methods, you should be able to resolve unresolved external symbol errors and successfully link your C++ projects.

Example Use Case

Suppose we have a project called MyProjectTest with a function multiple defined in function.cpp:

// function.h
#ifndef MY_FUNCTION_H
#define MY_FUNCTION_H

int multiple(int x, int y);

#endif

// function.cpp
#include "function.h"

int multiple(int x, int y) {
    return x * y;
}

We also have a unit test project UnitTest1 that references the multiple function:

// unittest1.cpp
#include "CppUnitTest.h"
#include "../MyProjectTest/function.h"

using namespace Microsoft::VisualStudio::CppUnitTestFramework;

namespace UnitTest1 {
    TEST_CLASS(UnitTest1) {
    public:
        TEST_METHOD(TestEqual) {
            Assert::AreEqual(multiple(2, 3), 6);
        }
    };
}

To resolve the unresolved external symbol error, we can create a static library from MyProjectTest and link it to UnitTest1. Alternatively, we can add a project reference between UnitTest1 and MyProjectTest.

By applying these methods, you should be able to successfully resolve unresolved external symbol errors and link your C++ projects.