g++ 编译.o目标文件出错，不知什么原因？ - 编程论坛

say.h

程序代码：

#include <iostream>
void sayhello(void);
class Say
{
        private:
                char * string;
        public:
                Say( char *  str)
                {
                        string  = str;
                }
                void sayThis(const char * str)
                {
                        std::cout << str << "from a static library\n";
                }
                void sayString(void);
};
~
~
~

-----------------------------------------
say.cpp

程序代码：

#include "say.h"
void Say::sayString()
{
        std::cout << string << "\n";
}

Say librarysay("library instance of Say");
~
~

--------------------------------------------
sayhello.cpp

程序代码：

#include "say.h"
void sayhello()
{
std::cout << "hello from a static library\n";
}

+++++++++++++++++++++++++++++++++++++++++++++
以下是报错：

Ubuntu 环境 g++

-virtual-machine:~/桌面/CLearning$ g++ say.cpp
say.cpp:7:41: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
Say librarysay("library instance of Say");
^
/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o：在函数‘_start’中：
(.text+0x20)：对‘main’未定义的引用

我想把 say.cpp 和 sayhello.cpp编译成目标文件，然后做成静态库，可是一编译就报错了，新手不知什么原因？？