如何在程序开始时声明 [英] How to declare at beginning of program

问题描述

在下面的清单中，尝试在main()函数之前声明矩形"r"的尝试将导致错误.

In the listing below, an attempt to declare the rectangle "r" before the main() function is called results in an error.

error: 'r' does not name a type r.x = 150;<br>

为什么必须在main()之后声明"r"?

Why must "r" be declared after main()?

#include <SDL2/SDL.h>

int main (int argc, char** argv) {
    // Creat a rect at pos ( 50, 50 ) that's 50 pixels wide and 50 pixels high.
    SDL_Rect r;
    r.x = 150;
    r.y = 150;
    r.w = 200;
    r.h = 100;

    SDL_Window* window = NULL;
    window = SDL_CreateWindow   ("SDL2 rectangle", SDL_WINDOWPOS_UNDEFINED,
                                 SDL_WINDOWPOS_UNDEFINED,
                                 640,
                                 480,
                                 SDL_WINDOW_SHOWN
    );

    // Setup renderer
    SDL_Renderer* renderer = NULL;
    renderer =  SDL_CreateRenderer( window, -1, SDL_RENDERER_ACCELERATED);
    SDL_SetRenderDrawColor( renderer, 0, 0, 0, 255 ); // black background
    SDL_RenderClear( renderer );    // Clear winow
    SDL_SetRenderDrawColor( renderer, 0, 255, 255, 255 ); // rgba drawing color

    // Render rect
    SDL_RenderFillRect( renderer, &r );

    // Render the rect to the screen
    SDL_RenderPresent(renderer);

    // Wait for 5 sec
    SDL_Delay( 5000 );

    SDL_DestroyWindow(window);
    SDL_Quit();

    return EXIT_SUCCESS;
}

推荐答案

r.x = 150;

这不是声明，也不是定义，而是赋值.

This is not a declaration, nor a definition, but an assignment.

C不允许在全局级别进行分配.

C does not allow assignments on global level.

您仍然可以在全局范围内定义变量

You still could define a variable at global scope

#include <SDL2/SDL.h>

SDL_Rect r;

int main (int argc, char** argv) {

每个全局定义的变量均接受默认初始化:

Every variable defined globally undergoes a default initialisation:

• integers变量设置为0.
• 浮点变量设置为0..
• 指针变量设置为NULL.

• integers variables are set to 0.
• floating point variables are set to 0..
• pointer variables are set to NULL.

甚至您还可以显式初始化

Even more you could also initialise it explicitly

#include <SDL2/SDL.h>

SDL_Rect r = {1, 2, 3, 4};

int main (int argc, char** argv) {

尽管初始化看起来与赋值相似，但并不相同(如您所见).

Although an initialisation looks similar to an assignment it is not the same (as you already observed).

在此处详细了解赋值和初始化之间的区别.

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