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。谢谢你的帮助

#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 - 我上面的引用来自clang，它有非常好的错误消息和对于初学者来说可能是一个很好的选择，因为那个。）

问题的长短是now在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，这可能会使您的生活学习时更容易。

Answer 2

wholeFunction返回局部变量的地址。

 struct fraction now = { (frac->numerator) - whole*(frac->denominator),
            (frac->denominator), whole};
 return &now;

以上导致未定义的行为。

Answer 3

C ++是一种具有价值语义的语言，它使语言与具有引用语义的Java或C＃有着截然不同的感觉。您可以在C ++中使用引用，但它们是显式的。您的代码存在许多问题，问题的核心是价值语义的概念。而不是单独检查每一个，我重构你的代码是更惯用的C ++。它绝不是完美的，但它已经删除了你的错误，并希望能为你提供一些关于如何用C ++做事的见解。

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

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

        return now;
    }

    return frac;
}

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

int isDivisible(const fraction& frac)
{
    double newNum = (double) frac.numerator;
    double newDen = (double) frac.denominator;

    for(int i = 0; i < 4; ++i)
    {
        if ( (newNum / toCheck[i]) == (frac.numerator / toCheck[i]) &&
                (newDen / toCheck[i]) == (frac.numerator / toCheck[i]))
            return toCheck[i];
    }
    return 0;
}

fraction simplifier(const fraction& frac)
{
    fraction new_frac = wholeNumber(frac);
    int factor = isDivisible(new_frac);
    while(factor)
    {
        new_frac.numerator = new_frac.numerator / factor;
        new_frac.denominator = new_frac.denominator / factor;
        int factor = isDivisible(new_frac);
    }

    return new_frac;
}

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

    std::cout << final.whole << " " << final.numerator << "/" << final.denominator;

    return 0;
}

c ++：结构中的int值正在改变值，没有明显的原因

3 个答案: