如何让构造函数与函数正常工作，使main()显示字符串和int数据并解决未解析外部符号错误？

Hey there! Let's figure out that unresolved external symbol error you're dealing with—this is a super common gotcha when working with C++ classes, and it has nothing to do with your void function not needing a return value (you're totally right about that part, by the way!).

What's causing the error? #

That "unresolved externals" message means your linker can't find the actual implementation of your showInfo() function (or maybe your constructor!). Usually this happens when:

• You declared the function inside your class, but never wrote the actual code for it outside the class without the proper scope resolution.
• You split your code into header and source files but forgot to compile/link the source file with your main code.

Let's fix this with a complete example #

Here's a working version that does exactly what you want: initializes a class with a string and int via constructor, then uses a void function to display those values.

Single-file implementation (easy for testing)

#include <iostream>
#include <string>

// Our class declaration
class MyData {
private:
    // Private member variables (initialized by constructor)
    std::string m_message;
    int m_number;
public:
    // Constructor declaration
    MyData(std::string msg, int num);
    
    // Void function declaration to display info
    void displayInfo();
};

// Constructor implementation (note the MyData:: scope!)
MyData::MyData(std::string msg, int num) 
    : m_message(msg), m_number(num) // Initialization list (more efficient than assignment)
{}

// HERE'S THE PART YOU PROBABLY MISSED!
// Implementation of the void display function, with proper scope
void MyData::displayInfo() {
    std::cout << "Stored String: " << m_message << "\n";
    std::cout << "Stored Integer: " << m_number << "\n";
}

int main() {
    // Create an instance of our class with sample data
    MyData myObj("Hello from my custom class!", 42);
    
    // Call the void function to display the data
    myObj.displayInfo();
    
    return 0;
}

If you're using separate header/source files

If you split your code into MyData.h and MyData.cpp, make sure:

1. MyData.h has the class declaration:

#pragma once
#include <string>

class MyData {
private:
    std::string m_message;
    int m_number;
public:
    MyData(std::string msg, int num);
    void displayInfo();
};

2. MyData.cpp has the implementations:

#include "MyData.h"
#include <iostream>

MyData::MyData(std::string msg, int num) : m_message(msg), m_number(num) {}

void MyData::displayInfo() {
    std::cout << "String: " << m_message << "\nInt: " << m_number << "\n";
}

3. When compiling, include both files:

g++ main.cpp MyData.cpp -o my_program

Key takeaways #

• Your understanding of void functions is correct—they don't return a value, so that's not the issue here.
• The linker error happens because you didn't provide the actual code for your member function(s) outside the class (or didn't link the file that has it).
• Always use ClassName:: when implementing class members outside the class declaration to tell the compiler which class the function belongs to.

内容的提问来源于stack exchange，提问作者SysAdminGrasshopper