问C++为什么使用参数化构造函数时竟然没有编译错误？

#include <string>
#include <iostream>

class MyException
{
    public:
        MyException(std::string s1) {std::cout << "MyException constructor, s1: " << s1 << std::endl;}
};

int main(){
    const char * text = "exception text";
    std::cout << "Creating MyException object using std::string(const char *)." << std::endl;
    MyException my_ex(std::string(text));
    std::cout << "MyException object created." << std::endl;
    //throw my_ex;

    std::string string_text("exception text");
    std::cout << "Creating MyException object using std::string." << std::endl;
    MyException my_ex2(string_text);
    std::cout << "MyException object created." << std::endl;
    // throw my_ex2;

    return 0;
}

 $ g++ main.cpp
 $ ./a.out
Creating MyException object using std::string(const char *).
MyException object created.
Creating MyException object using std::string.
MyException constructor, s1: exception text
MyException object created.

throw my_ex;

 $ g++ main.cpp
/tmp/ccpWitl8.o: In function `main':
main.cpp:(.text+0x55): undefined reference to `my_ex(std::string)'
collect2: error: ld returned 1 exit status

const char * text = "exception text";
std::cout << "Creating MyException object using std::string(const char *)." << std::endl;
MyException my_ex((std::string(text)));
std::cout << "MyException object created." << std::endl;
throw my_ex;

 $ g++ main.cpp
 $ ./a.out
Creating MyException object using std::string(const char *).
MyException constructor, s1: exception text
MyException object created.
terminate called after throwing an instance of 'MyException'
Aborted (core dumped)