c++ - C++11 类型检查代码的奇怪编译器警告

Question

我正在编写一个带有进位/溢出检查的通用加法器，并且我大量使用 c++11 类型检查功能。

这是我的代码:

#include <iostream>
using namespace std;
#define MIN_OF(TYPE) ( (std::is_signed<decltype(res)>::value) ? \
                                (1 << ( ( sizeof(decltype(res)) * 8 ) - 1)) : \
                                0 )

#define MAX_OF(TYPE) (~MIN_OF(TYPE))


#define ABS(x)  (x < 0 ? -x : x)

class Flags
{
public:
    void setSign(bool x)
    {
        cout << boolalpha;
        cout << "setSign: " << x << endl;
    }
    void setOverflow(bool x)
    {
        cout << boolalpha;
        cout << "setOverflow: " << x << endl;
    }
    void setCarry(bool x)
    {
        cout << boolalpha;
        cout << "setCarry: " << x << endl;
    }
    void setZero(bool x)
    {
        cout << boolalpha;
        cout << "setZero: " << x << endl;
    }
};

template <typename TYPE, TYPE def>
class Value
{
public:
static inline TYPE get()
{
    return def;
}
static inline void set(TYPE x)
{
    cout << "value: " << hex << x << endl;
}
};


template <class A, class B, class RES>
struct ADD
{
static void Do(Flags* _flags)
{
    if (std::is_convertible<decltype(A::get()),decltype(RES::get())>::value)
    {
        decltype(A::get()) _a = A::get();
        decltype(B::get()) _b = B::get();

        decltype(RES::get()) res = _a;

        if (_b != 0)
        {
            res = res + _b;

            if (std::is_signed<decltype(res)>::value)
            {
                unsigned char highestbit_a = static_cast<unsigned char>(0x1 & (_a >> (( sizeof(decltype(_a)) * 8 ) - 1)));
                unsigned char highestbit_b = static_cast<unsigned char>(0x1 & (_b >> (( sizeof(decltype(_b)) * 8 ) - 1)));
                unsigned char highestbit_res = static_cast<unsigned char>(0x1 & (res >> (( sizeof(decltype(res)) * 8 ) - 1)));

                _flags->setSign( (res < 0) );
                _flags->setOverflow( ((highestbit_a & highestbit_b) != highestbit_res) );
            }
            else
            {
                _flags->setSign( false );
                _flags->setOverflow( false );
            }

            bool setCarryFlag = false;

            if (std::is_signed<decltype(_b)>::value)
            {
                if(_b < 0)
                {
                    /* as _b is negative, we add _b to lowest_res, if the result
                     *  is greater as _a, _a + _b (with _b as negative number) would
                     * result in an carry out
                     */
                    setCarryFlag = (static_cast<decltype(_a)>(ABS((MIN_OF(decltype(res)) - _b))) > _a);
                }
                else
                {
                    setCarryFlag = (static_cast<decltype(_a)>((MAX_OF(decltype(res)) - _b)) < _a);
                }
            }
            else
            {
                //get difference of one summand to results highest until carry
                /* MARKED LINE: this branch gets wrongly checked */
                setCarryFlag = ((MAX_OF(decltype(res)) - _b) < _a);
            }

            _flags->setCarry( setCarryFlag );
        }
        else
        {
            if (std::is_signed<decltype(res)>::value)
            {
                _flags->setSign( (res < 0) );
            }
        }

        _flags->setZero( (res == 0) );

        //store result
        RES::set(res);
    }
}
};



int main()
{
Flags* f = new Flags();
ADD<Value<unsigned int, 1>, Value<signed int, 6>, Value<unsigned int, 1>>::Do(f);

return 0;
}

问题出现在“MARKED LINE:”处。通常，我会理解编译器不会使用这个分支，因为 _b 是 signed int 类型，所以 is_signed 应该是 true，所以编译器应该只使用 if 分支中的 whats 并丢弃 else 分支。但它似乎没有这样做，因为我收到警告:

 warning: comparison between signed and unsigned integer expressions [-Wsign-compare]|

指向这一行。但这不是我想要的。有什么想法可以告诉编译器做正确的事吗？

编译器是:gcc 4.7.2 on x86-64, debian

谢谢!

Answer 1

这是警告，不是错误。警告是为了警告您可能不想做但不合法的事情。在这种情况下你想这样做，所以忽略它。您可以使用 pragma 来忽略警告，但我建议记录您如何知道它对 future 的开发人员是安全的。

用于禁用此警告的 GCC 特定编译指示是:<​​/p>

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wsign-compare"
setCarryFlag = ((MAX_OF(decltype(res)) - _b) < _a);
#pragma GCC diagnostic pop