链接器命令在 macos 上失败，退出代码为 1（使用 -v 查看调用）

Question

I am practicing a simple c++ code base on example

class Tset{
    public:
        Tset();
        Tset operator+( const Tset& a);

};

Tset Tset::operator+(const Tset& a){
    Tset t;
    return t;
}

but when I use g++ to compile this code it occur this error

Mac Desktop % g++ hw2.cpp
Undefined symbols for architecture x86_64:
  "Tset::Tset()", referenced from:
      Tset::operator+(Tset const&) in hw2-43d108.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Can anybody tell me what's wrong?

here is my g++ version:

Mac Desktop % g++ -v     
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 11.0.0 (clang-1100.0.33.12)
Target: x86_64-apple-darwin19.0.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Answer 1

The problem is that you have not provided a definition of the constructor.

This function is implicitly calling Tset::Tset() , the constructor:

Tset Tset::operator+(const Tset& a){
    Tset t;    // Requires definition of constructor.
    return t;
}

But in your class, you do not define the constructor:

class Tset {
public:
    Tset();            // No definition.  :-(
    Tset operator+(const Tset& a);
};

You can fix this by providing a definition of the constructor, or defaulting the constructor:

class Tset {
public:
    Tset() = default;
    Tset operator+( const Tset& a);
};