Breaking out of a for-loop is a little strange to me, since the semantics of a for-loop typically indicate that it will execute a specified number of times. However, it's not bad in all cases; if you're searching for something in a collection and want to break after you find it, it's useful. Breaking out of nested loops, however, isn't possible in C++; it is in other languages through the use of a labeled break. You can use a label and a goto, but that might give you heartburn at night..? Seems like the best option though.

打破嵌套循环的另一种方法是将两个循环分解成一个单独的函数，并在希望退出时从该函数返回。

当然，这也引发了另一个争论，即是否应该显式地从函数的任何地方返回，而不是在函数的末尾。

你可以使用“goto”来留下嵌套循环 下面是我的原始代码，包括“goto”

int main()
{
    string str;
    while (cin >> str)
    {
        if (str == "0")
            break;
        int sum = 0;
        for (auto ch : str)
        {
            if (ch <= 'z' && ch >= 'a')
                sum += (ch - 'a' + 1);
            else if (ch >= 'A' && ch <= 'Z')
                sum += (ch - 'A' + 1);
            else
            {
                cout << "Fail" << endl;
                goto fail;
            }
        }

        cout << sum << endl;
        fail:
    }
    return 0;
}

然而，我可以通过添加函数“计算”来避免“goto”

void calculate(const string &str)
{
    int sum = 0;
    for (auto ch : str)
    {
        if (ch <= 'z' && ch >= 'a')
            sum += (ch - 'a' + 1);
        else if (ch >= 'A' && ch <= 'Z')
            sum += (ch - 'A' + 1);
        else
        {
            cout << "Fail" << endl;
            return;
        }
    }

    cout << sum << endl;
}

int main()
{
    string str;
    while (cin >> str)
    {
        if (str == "0")
            break;
        calculate(str);
    }
    return 0;
}

break语句终止它出现的最近的封闭do、for、switch或while语句的执行。控制权转移到终止语句后面的语句。

从msdn。

我不确定这样做是否值得，但你可以用一些简单的宏来模拟Java的命名循环:

#define LOOP_NAME(name) \
    if ([[maybe_unused]] constexpr bool _namedloop_InvalidBreakOrContinue = false) \
    { \
        [[maybe_unused]] CAT(_namedloop_break_,name): break; \
        [[maybe_unused]] CAT(_namedloop_continue_,name): continue; \
    } \
    else

#define BREAK(name) goto CAT(_namedloop_break_,name)
#define CONTINUE(name) goto CAT(_namedloop_continue_,name)

#define CAT(x,y) CAT_(x,y)
#define CAT_(x,y) x##y

使用示例:

#include <iostream>

int main()
{
    // Prints:
    // 0 0
    // 0 1
    // 0 2
    // 1 0
    // 1 1

    for (int i = 0; i < 3; i++) LOOP_NAME(foo)
    {
        for (int j = 0; j < 3; j++)
        {
            std::cout << i << ' ' << j << '\n';
            if (i == 1 && j == 1)
                BREAK(foo);
        }
    }
}

另一个例子:

#include <iostream>

int main()
{
    // Prints: 
    // 0
    // 1
    // 0
    // 1
    // 0
    // 1

    int count = 3;
    do LOOP_NAME(foo)
    {
        for (int j = 0; j < 3; j++)
        {
            std::cout << ' ' << j << '\n';
            if (j == 1)
                CONTINUE(foo);
        }
    }
    while(count-- > 1);
}

Breaking out of a for-loop is a little strange to me, since the semantics of a for-loop typically indicate that it will execute a specified number of times. However, it's not bad in all cases; if you're searching for something in a collection and want to break after you find it, it's useful. Breaking out of nested loops, however, isn't possible in C++; it is in other languages through the use of a labeled break. You can use a label and a goto, but that might give you heartburn at night..? Seems like the best option though.