c ++:结构中的int值正在改变值,没有明显的原因

Question

我在学习java之后想学习c ++.我被告知结构很像java中带有实例变量的类.分数结构中的"整数"值似乎在isDivisible函数中从1随机变为零.为什么会这样？请不要遗漏细节.谢谢您的帮助

#include <stdio.h>
#include <math.h>
#include <iostream>
using namespace std;

#define LARGEST(a,b) ((a > b) ? 1 : 0)

struct fraction {
    int numerator;
    int denominator;
    int whole;
};

fraction* wholeNumber(fraction * frac){
    if (LARGEST((frac->numerator), (frac->denominator)) == 1){
        double w = (((double) (frac->numerator)) / ((double) (frac->denominator)));
        int whole = floor(w);
        struct fraction now = { (frac->numerator) - whole*(frac->denominator),
                (frac->denominator), whole};
        return &now;
    }

    return frac;
}

int toCheck[4] = {2,3,5,7}; 

int isDivisible(fraction * frac){
    double newNum = (double) frac->numerator;
    double newDen = (double) frac->denominator;
    // The value of whole seems to change right about here after the two doubles are declared
    for(int i = 0; i < 4; ++i){
        if (( (newNum / toCheck[i]) == (frac->numerator / toCheck[i]) ) ? true : false &&
                ((newDen / toCheck[i]) == (frac->numerator / toCheck[i]) ) ? true : false)
            return toCheck[i];
    }
    return 0;
}

fraction* simplifier(fraction * frac){
    frac = wholeNumber(frac);
    while(isDivisible(frac) != 0){
        int factor = isDivisible(frac);
        frac->numerator = frac->numerator / factor;
        frac->denominator = frac->denominator / factor;
    }

    return frac;
}

int main (int argc, char ** argv){
    struct fraction frac = {55, 50, 0}; // fraction declared, 0 55/50
    struct fraction * final = simplifier(&frac);

    printf("%d %d/%d\n", final->whole, final->numerator, final->denominator);

    return 0;
}

Answer 1

编译时是否收到此警告(或类似内容)？

example.cpp:20:17: warning: address of stack memory associated with local
      variable 'now' returned [-Wreturn-stack-address]
        return &now;
                ^~~

返回指向堆栈变量的指针是个坏消息.如果您的编译器没有警告您,您需要打开一些警告标志(或者获得更好的编译器 - 我的引用来自clang,它有非常好的错误消息,对于初学者来说可能是一个不错的选择,因为这样).

你的问题的长期和短期是now当函数结束时超出范围.这意味着访问返回的指针会导致未定义的行为.

您可能希望在程序中使用更少的指针 - 如果您使用的是C++,则可以访问引用,但除此之外,C和C++允许结构按值传递,这可能会使您在学习时更轻松.