How to solve C++ compilation error: 'undefined reference to 'class::function'?

Solution to C compilation error: 'undefined reference to 'class::function', how to solve it?

When writing C programs, we often encounter compilation errors. One of the common errors is the "undefined reference to 'class::function'" error. This error is generally caused by a function or member of the class being referenced in the C source file, but the corresponding function definition or implementation was not found during the compilation process. In this article, we will discuss the causes of this error and give some ways to fix it.

Before we start to solve this error, we need to understand some knowledge related to the C compilation process. Compilation of C is divided into two steps: compilation and linking. During the compilation process, the compiler checks the code in the source file and generates the corresponding object file. During the linking process, the compiler connects the object files and other library files together to generate an executable file.

When we call a function or member of a class in a C source file, the compiler will generate the corresponding object file during the compilation process and parse the call during the linking process. However, if the definition or implementation of this function cannot be found during the linking process, an 'undefined reference to 'class::function' error will occur.

So, how should we solve this error? Several possible solutions are listed below:

1. Ensure that the definition or implementation of the function exists in the source file:

First, we need to check the definition of the function of the class or whether the implementation exists in the source file. If this function is not defined or implemented correctly, the compiler cannot find it, resulting in a link error.

The sample code is as follows:

// MyClass.h
class MyClass {
public:
    void myFunction();
};

// MyClass.cpp
#include "MyClass.h"
void MyClass::myFunction() {
    // Function implementation
}

In this example, we declare the class MyClass in the header file "MyClass.h" and implement the function in the source file "MyClass.cpp" myFunction.

2. Check the namespace of the function:

If our class is defined in a namespace, then when calling the function of the class, we need to specify the name to which the function belongs space. Otherwise, the compiler will consider the function to be a global function rather than a member function of the class.

The sample code is as follows:

// MyClass.h
namespace MyNamespace {
    class MyClass {
    public:
        void myFunction();
    };
}

// MyClass.cpp
#include "MyClass.h"
void MyNamespace::MyClass::myFunction() {
    // Function implementation
}

In this example, we define the class MyClass in the namespace "MyNamespace". When calling the function myFunction, you need to use the "MyNamespace::MyClass" syntax.

3. Check whether the declaration and definition of the function match:

If the parameter list of the function in the class does not match the declaration and definition, the compiler will also report an error. When calling a function, ensure that the parameters passed match the type, order, and number of parameters in the function definition.

The sample code is as follows:

// MyClass.h
class MyClass {
public:
    void myFunction(int x, int y);
};

// MyClass.cpp
#include "MyClass.h"
void MyClass::myFunction(int x, int y) {
    // Function implementation
}

In this example, the function myFunction accepts two integer parameters. When calling a function, you need to pass the same number and type of parameters.

To sum up, solving the 'undefined reference to 'class::function'' error requires us to carefully check whether the definition or implementation of the function exists, whether the namespace to which the function belongs is correct, and whether the declaration and definition of the function match. . By defining and implementing the function correctly, and ensuring that the parameters are passed correctly when calling, we can avoid this compilation error.

I hope this article can help readers solve similar compilation errors and improve the efficiency and quality of writing C programs.

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