编译时未检测到主要功能

我正在尝试运行一个打开窗口的程序。目的是让程序开始打开一个窗口是所有程序的开始是正确的吗？

但是，当我出于某种原因运行代码时，我会遇到此错误：

Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

但是，在我的代码中，我确实具有main()函数，所以为什么要遇到此错误？

这是我的代码：

#include <SDL2/SDL.h>
#include <SDL2_image/SDL_image.h>
#include <SDL2_ttf/SDL_ttf.h>
#include <stdio.h>
int main(){
    if(SDL_Init( SDL_INIT_EVERYTHING ) < 0){
        std::cout << "error 1n";
        std::cout << SDL_GetError();
        std::cout << "n";
        return -1;
        }
    if(TTF_Init() < 0){
        std::cout << "error 2n";
        std::cout << TTF_GetError();
        std::cout << "n";
        return -1;
    }
    SDL_Window* window = SDL_CreateWindow("test", 0, 0, 500, 500, 0);
    if(!window){
        std::cout << "error 3n";
        std::cout << SDL_GetError();
        std::cout << "n";
        return -1;
    }
    int windowid = SDL_GetWindowID(window);
    SDL_Renderer* Renderer = SDL_CreateRenderer(window, -1, 0);
    running = true;
    SDL_Event event;
    while(running){
        while(SDL_PollEvent(&event)){
            if(event.type == SDL_WindowEvent){
                if(event.window.windowID == windowid){
                    if(event.window.type == SDL_WindowClose){
                        Destroywindow(window);
                        running = false;
                    }
                }
            }
        }
    }
    return 0;
}

我的制作文件看起来像这样：

#!/bin/bash
brew update
brew install sdl2
g++ -o /Users/mikahshattuck/noneproject/none2019-05-0909-22- 
14:2:/none.app/Contents/MacOS/mainrun.cpp -I /Library/Frameworks -l 
SDL2
exit 0

这是完整的：

Already up-to-date.
Warning: sdl2 2.0.9_1 is already installed and up-to-date
To reinstall 2.0.9_1, run `brew reinstall sdl2`
Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see 
invocation)
logout
Saving session...
...copying shared history...
...saving history...truncating history files...
...completed.
[Process completed]

预先感谢您