c++ - 请求从 'int' 到非标量类型 conts 'Vec2' 的转换

Question

我是 C++ 的初学者，我正在尝试做一条蛇，但我遇到了一个问题。

错误行:

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()

那么我该如何解决这个问题呢？

Answer 1

const Vec2 ARENA_POSITION = (0, 50);

不是 aggregate initialization 的语法.这是一个带括号的 comma operator ，它丢弃 0(关于逗号运算符的操作数没有任何效果的警告)并尝试做

const Vec2 ARENA_POSITION = 50;

---

正确的语法是以下两者之一:

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