C++:添加逻辑上正确的 if 语句会使我的程序崩溃

Question

我是一个新手，我遇到了一个似乎无法解决的问题。也就是说，我有这两个独立的工作代码块，但如果我同时使用它们，程序就会崩溃(返回错误代码 3221225477)。

#include <conio.h>
#include <iostream>
#include <string>
#include <windows.h>
#include <chrono>
#include <thread>
#include <vector>
#include <time.h>

#define KEY_UP 72
#define KEY_DOWN 80
#define KEY_LEFT 75
#define KEY_RIGHT 77
#define ENTER 13

using namespace std;

struct set {
int speed=0;
bool obs=0;
} settings;

void gotoyx( int y, int x ) { //moves the console cursor
COORD coord;
coord.X = x;
coord.Y = y;
SetConsoleCursorPosition(GetStdHandle( STD_OUTPUT_HANDLE ), coord);
}

void setup() { //used to determine the snake's speed, expressed as delay between movement
int c = 0;
string o1=">SLOW", o2=" MEDIUM", o3=" FAST";
while(settings.speed==0)
{
    c = 0;
    gotoyx(c, c); //sets cursor at top-left
    cout << "Please choose your speed setting:\n"<< o1 << endl << o2 << endl << o3 << endl;
    switch((c=getch())) {
    case KEY_UP:
        if(o2.at(0)=='>'){
            o1[0]='>';
            o2[0]=' ';
        }
        else if(o3.at(0)=='>'){
            o2[0]='>';
            o3[0]=' ';
        }
        break;
    case KEY_DOWN:
        if(o1.at(0)=='>'){
            o2[0]='>';
            o1[0]=' ';
        }
        else if(o2.at(0)=='>'){
            o3[0]='>';
            o2[0]=' ';
        }
        break;
    case ENTER:
        if(o1.at(0)=='>') {
            cout << "You chose SLOW.\n";
            settings.speed=160; 
        }
        else if(o2.at(0)=='>') {
            cout << "You chose MEDIUM.\n";
            settings.speed=120;
        }
        else {
            cout << "You chose FAST.\n";
            settings.speed=80;
        }
        settings.obs=1;
        cout << "Press a key to continue.";
        getch();
        break;
    default:
        break;
    }
}
}

void draw_border() {
for(int i=0; i<80; i++){ //80 fills screen vertically - TOP
    cout<<"#";
}
for(int i=0; i<23; i++){ //25 fills screen horizontally (2 used for top and bottom)
    cout<<"#";
    for(int i=0; i<78; i++){
    cout<<" ";
    }
    cout<<"#";
}
for(int i=0; i<80; i++){ //80 fills screen vertically - BOTTOM
    cout<<"#";
}
}

void ShowConsoleCursor(bool showFlag) { //blinking-underscore-with-console
HANDLE out = GetStdHandle(STD_OUTPUT_HANDLE);

CONSOLE_CURSOR_INFO     cursorInfo;

GetConsoleCursorInfo(out, &cursorInfo);
cursorInfo.bVisible = showFlag; // set the cursor visibility
SetConsoleCursorInfo(out, &cursorInfo);
  }

int keypad(int out){                                        //reads arrow keys during game
    if (GetAsyncKeyState(VK_UP) & 0x8000 && out!=1 ) {  //MSB set if currently pressed
        out=0;                                          //if nothing is pressed it returns
    }                                                   //the last pressed button
    else if (GetAsyncKeyState(VK_DOWN) & 0x8000 && out!=0) {
        out=1;
    }
    else if (GetAsyncKeyState(VK_LEFT) & 0x8000 && out!=3) {
        out=2;
    }
    else if (GetAsyncKeyState(VK_RIGHT) & 0x8000 && out!=2) {
        out=3;
    }
    return out;                                         
}

int main() {
ShowConsoleCursor(false);                               //disable blinking console cursor
setup();
int tmp=0, cntr_g=0, cntr_o=0;
system("cls");                                          //clear screen after inital setup
draw_border();
gotoyx(tmp, tmp);                                       //using cursor to scroll to console top
vector<vector<int>> snake_pos;                          //tracks snake location
snake_pos.emplace(snake_pos.begin(), std::initializer_list<int>{13,40});
int y = snake_pos[0][0];                                //Y coordinate of snake head
int x = snake_pos[0][1];                                //X coordinate of snake head
gotoyx(y, x);
cout<<"*";
int key, boost_y, boost_x, obstacle_y, obstacle_x;
bool b_flag=0;
vector<vector<int>> boost (1, vector<int>(2, 0));
vector<vector<int>> obstacle (1, vector<int>(2, 0));
while(1){
    key=keypad(key);
    if (key==0) {
        snake_pos.emplace(snake_pos.begin(), std::initializer_list<int> {y-1, x});
    }
    else if (key==1) {
        snake_pos.emplace(snake_pos.begin(), std::initializer_list<int> {y+1, x});
    }
    else if (key==2) {
        snake_pos.emplace(snake_pos.begin(), std::initializer_list<int> {y, x-1});
    }
    else if (key==3) {
        snake_pos.emplace(snake_pos.begin(), std::initializer_list<int> {y, x+1});
    }

    y = snake_pos[0][0]; x = snake_pos[0][1];           //updating snake head coordinates

    if (y==0 | y==24 | x==0 | x==80 ) {                 //checking whether player hit wall
        return 0;
    }
    for (tmp=1; tmp!=snake_pos.size()-1; ++tmp) {       //checking whether player hit self
        if (y==snake_pos[tmp][0] && x==snake_pos[tmp][1]) {
            return 0;
        }
    }

    //check if obstacle was hit [not here yet]

    ++cntr_g;                                           //generating growth boost block
    if (cntr_g==settings.speed/5) {
        label_growth:
        srand(time(NULL));
        boost_x=(rand()%78)+1;
        boost_y=(rand()%23)+1;
        for (tmp=0; tmp!=snake_pos.size()-1; ++tmp) {
            if (boost_y==snake_pos[tmp][0] && boost_x==snake_pos[tmp][1]) {
                goto label_growth;
            }
        }
        gotoyx(boost_y, boost_x); cout<<"O";
        boost.emplace_back(std::initializer_list<int> {boost_y, boost_x});
        cntr_g=0;
    }

    /*
    ++cntr_o;                                               //generating obstacle block
    if ((cntr_o==settings.speed/4) && (settings.obs==1)) {  //also check if obs enabled
        label_obstacle:
        srand(time(NULL));
        obstacle_x=(rand()%78)+1;
        obstacle_y=(rand()%23)+1;
        for (tmp=0; tmp!=snake_pos.size()-1; ++tmp) {       //check overlap with snake
            if (obstacle_y==snake_pos[tmp][0] && obstacle_x==snake_pos[tmp][1]) {
                goto label_obstacle;
            }
        }
        for (tmp=0; tmp!=boost.size()-1; ++tmp) {           //check overlap with boosts
            if (obstacle_y==boost[tmp][0] && obstacle_x==boost[tmp][1]) {
                goto label_obstacle;
            }
        }
        gotoyx(obstacle_y, obstacle_x); cout<<"X";
        obstacle.emplace_back(std::initializer_list<int> {obstacle_y, obstacle_x});
        cntr_o=0;
    }*/

    gotoyx(y, x); cout<<"*"; b_flag=0;                  //updating graphics for new snake
    for (tmp=0; tmp!=boost.size()-1; ++tmp) {           
        if (y==boost[tmp][0] && x==boost[tmp][1]) {
            boost.erase(boost.begin()+tmp);
            b_flag=1;
            break;
        }
    }
    if (b_flag==0){                                     //if snake didn't grow, last block stays
        gotoyx(snake_pos.back()[0], snake_pos.back()[1]); cout<<" ";
        snake_pos.pop_back();
    }

    std::this_thread::sleep_for(std::chrono::milliseconds(settings.speed));
}

return 0;
}

也就是说，问题出在生长 block 部分和障碍 block 部分的 if 语句上。当其中一个被注释掉时，程序(游戏)将按预期工作，但如果将它们都保留在其中，则不会。事实上，即使只是留在 cntr_o++; 中而没有以下 if 语句足以使其崩溃。

Answer 1

编译器是善变的东西。由于编译器偶然布置内存的方式，您的代码可能不正确，但仍然不会崩溃。这是“未定义行为”的结果。未定义的行为可能会导致程序崩溃，但也可能没有明显的影响。

虽然您的代码对于随意调试来说有点太杂乱，但您可能会发现调试器可能会有所帮助。如果您能够使用 clang，我建议添加 -Wall -Wextra 和 -fsanitize=address -fsanitize=undefined 来捕获一些更简单的问题。