我应该将结构用作 C++ 中的参数化构造函数吗？

Question

This is my sample code. I used it like this, but I had an error from //error-line .

The following error appeared:

Variable has incomplete type 'std::TestCpp:: TestString'

class TestCpp{
    public:
        any value;
        struct TestString;

        TestCpp(TestString Value){ // error
            this->value = static_cast<any>(Value);
            cout << Value;
        }

};

Answer 1

You are using an instance of TestString in the constructor of TestCpp before it has been defined. You should first define TestString , then define the constructor of TestCpp . My guess is you don't want to define TestString where you declared it. To achieve that, you can do this by defining the constructor of TestCpp in a source file for instance.

// header file
struct TestCpp
{
    std::any _value;
    struct TestString;
    TestCpp(TestString); // declare here, define in source file    
};

// source file
#include "header_file"

struct TestCpp::TestString { /* ... */ };

TestCpp::TestCpp(TestString) { /* ... */ }