C++(看似)随机编译器错误

Question

多亏了我在 Oxfam 书店找到的一本小书和一本大书，我一直在研究 C、C++ 和 Allegro。我现在理解得很好，但我遇到了困难……每当我编译时，我都会遇到这些错误:

archiboldian@archiboldian:~/Documents/C++ Projects/particles$ g++ particles.c -lalleg -lnoise -o particles
particles.c:19: error: array bound is not an integer constant before ‘]’ token
particles.c:20: error: ‘Vector2D’ does not name a type
particles.c:21: error: ‘Vector2D’ does not name a type
particles.c: In function ‘int main()’:
particles.c:26: error: ‘nPos’ was not declared in this scope
particles.c:28: error: ‘nVel’ was not declared in this scope
particles.c:29: error: ‘nvel’ was not declared in this scope
particles.c:31: error: ‘addParticle’ was not declared in this scope
particles.c: At global scope:
particles.c:47: error: ‘Vector2D’ has not been declared
particles.c:47: error: ‘Color’ has not been declared
particles.c: In function ‘void addParticle(int, int, Vector2d, int, int, int)’:
particles.c:50: error: ‘particles’ was not declared in this scope

这是我的代码...

#include "allegro.h"

struct Vector2d{
    double x;
    double y;
};

struct Particle {
    Vector2d Pos;
    Vector2d Vel;
    int age;
    int LifeSpan;
    int colour;
    int size;
};

int max = 50;
int pcount = 0;
Particle particles[max];

int main(void) {

    Vector2D nPos;
    Vector2D nVel;

    nPos.x = 320;
    nPos.y = 240;
    nVel.x = 2;
    nvel.y = 0;

    addParticle(10, nPos, nVel, 20, makecol(255,255,255), 2);

    allegro_init();
    install_keyboard();

    set_gfx_mode(GFX_AUTODETECT_WINDOWED, 640, 480, 0, 0);

    while(!key[KEY_ESC]) {
        for(int i=0;i<pcount;i++){

        }
    }

    allegro_exit();
}

void addParticle(int addp, Vector2D Pos, Vector2d Vel, int LifeSpan, Color colour, int size) {
    for(int i=0;i<addp;i++){
        pcount++;
        particles[pcount].Pos = Pos;
        particles[pcount].Vel = Vel;
        particles[pcount].LifeSpan = LifeSpan;
        particles[pcount].colour = colour;
        particles[pcount].size = size;
    }
}

END_OF_MAIN();

根据我从调试输出中收集到的信息，第一个错误是关于'粒子粒子[最大];'行和消息听起来好像在'particles'末尾有这个'[max]'是错误的，但直到现在它工作正常并且编译没有问题。这可能只是一个打字错误或误解或其他什么，但我真的想不通。

如您所见，这是对粒子系统的尝试，以及任何关于改进的提示(这是一个词吗？)我的代码非常感谢:)

谢谢。

Answer 1

对于能够用作数组大小的变量，它必须是常量表达式。这在 C++ 中用 const 表示。在 C 中，您将使用 #define。

// C++
const int MAX = 50;
/* C */
#define MAX 50
/* both C & C++ */
enum { MAX = 50 };
Particle particles[MAX];