从'int'转换为非标量类型conts'Vec2'请求

Question

我是C++的初学者，我试图做一条蛇，但是我遇到了问题。

行错误：

C:\Users\Kcper\Desktop\Snej w allegro\main.cpp||In function 'void display_game()':| 
C:\Users\Kcper\Desktop\Snej w allegro\main.cpp|58|warning: left operand of comma operator has no effect [-Wunused-value]| 
C:\Users\Kcper\Desktop\Snej w allegro\main.cpp|58|error: conversion from 'int' to non-scalar type 'const Vec2' requested| 

这里是一个代码：

#include <allegro.h> 
    #include <winalleg.h> 
    #define TICS_PER_SECOND 50 
    #define MAX_FRAMESKIP 10 
    #define MAX_MENU_STATES 4 
    const int SKIP_TICKS = 1000/TICS_PER_SECOND; 

    BITMAP * buffer; 

    struct Vec2 
    { 
     int x; 
     int y; 
    }; 

    enum States 
    { 
     state_menu, 
     state_game, 
     state_continue, 
     state_exit, 
     state_leaderboard, 
    }; 

    enum Positions 
    { 
     pos_game=30, 
     pos_continue=50, 
     pos_exit=70, 
     pos_leaderboard=90, 
    }; 

    States menu(int *state) 
    { 
     int value; 
     if (keypressed()) 
     { 
      value=readkey(); 
      switch(value>>8) 
      { 
      case KEY_DOWN: 
       if(*state<MAX_MENU_STATES)(*state)++; 
       break; 
      case KEY_UP: 
       if(*state>1)(*state)--; 
       break; 
      case KEY_ENTER: 
       return States(*state); 
      } 
     } 
     return state_menu; 
    } 

    void display_game() 
    { 
     int color_zielony=makecol(0,255,0); 
     int color_czerwony=makecol(255,0,0); 
     const Vec2 ARENA_POSITION=(0,50); //<---- There is a problem 
     rect(buffer,ARENA_POSITION.x,ARENA_POSITION.y,SCREEN_H-1,SCREEN_W-1,color_czerwony); 
     textout_centre_ex(buffer,font,"Carrotz: ",SCREEN_W/2,SCREEN_H/2,color_zielony,0); 
    } 

    void display_menu(States state) 
    { 
     int color_bialy=makecol(255,255,255); 
     int color_czerwony=makecol(255,0,0); 
     int color_niebieski=makecol(0,0,255); 
     int color_zielony=makecol(0,255,0); 

     textout_centre_ex(buffer,font,"SNAKE by Kacper",SCREEN_W/2,10,color_niebieski,0); 
     textout_centre_ex(buffer,font,"New Game",SCREEN_W/2,30,color_bialy,2); 
     textout_centre_ex(buffer,font,"Leadrboard",SCREEN_W/2,70,color_bialy,2); 
     textout_centre_ex(buffer,font,"Continue",SCREEN_W/2,50,color_bialy,2); 
     textout_centre_ex(buffer,font,"Exit",SCREEN_W/2,90,color_czerwony,2); 

     int position; 
     switch(state) 
     { 
      case state_game: 
      position=pos_game; 
      break; 
      case state_continue: 
      position=pos_continue; 
      break; 
      case state_leaderboard: 
       position=pos_leaderboard; 
       break; 
      case state_exit: 
       position=pos_exit; 
       break; 
     } 
     rectfill(buffer,SCREEN_W/2-100,position,SCREEN_H-180,position+10,color_zielony); 
    } 

    inline void init() 
    { 
     allegro_init(); 
     set_color_depth(32); 
     set_gfx_mode(GFX_AUTODETECT_WINDOWED,760,480,0,0); 

     install_timer(); 
     install_keyboard(); 
     install_mouse(); 
    } 

    inline void deinit() 
    { 
     destroy_bitmap(buffer); 
     clear_keybuf(); 
     allegro_exit(); 
    } 

    int main() 
    { 
    init(); 
    States currentState=state_menu; 
    int currentMenuState=1; 
    int next_game_tick = GetTickCount(); 
    int loops; 
    float interpolation; 
    buffer=create_bitmap (SCREEN_W,SCREEN_H); 
    while (!key[KEY_ESC]) 
    { 
     loops=0; 
     while (GetTickCount()>next_game_tick&&loops<MAX_FRAMESKIP) 
     { 
     loops++; 
     next_game_tick+=SKIP_TICKS; 
     switch (currentState) 
     { 
      case state_menu: 
      currentState=menu(&currentMenuState); 
      break; 
      case state_game: 
      break; 
     } 
     } 

     clear(buffer); 
     switch (currentState) 
     { 
      case state_menu: 
      display_menu(States(currentMenuState)); 
      break; 
      case state_game: 
       display_game(); 
      break; 
     } 
     blit(buffer,screen,0,0,0,0,SCREEN_W,SCREEN_H); 

    } 
     deinit(); 
     return 0; 
    } 
    END_OF_MAIN() 

那么如何才能解决这个问题呢？

Answer 1

const Vec2 ARENA_POSITION = (0, 50); 

不是aggregate initialization的语法。这是相当一个括号comma operator，它丢弃0（警告关于没有任何效果的逗号操作符的操作数），并尝试做

const Vec2 ARENA_POSITION = 50; 

---

正确的语法或者是两个：

const Vec2 ARENA_POSITION = {0, 50}; 
const Vec2 ARENA_POSITION{0, 50};