c++的float(NaN)

经常在AS3里面收获到NaN,但一直认为C++是弱类型的，只管内存，再加上平时都跟uint32打交道比较多，

才会在今天踩到陷阱。碰到一个值为-nan(0x400000)造成的crash.

google了一下收获到：

http://stackoverflow.com/questions/570669/checking-if-a-double-or-float-is-nan-in-c

for a float f, f != f will be true only if f is NaN.

对于float类型变量f,f != f 仅在f 是 NaN时成立。

看来判断float是否为有效值是还得多一个判断f==f

直接看代码:

#include <stdio.h>

int main(){
    unsigned int i = 0x400000;
    float f = 0.0f;
    printf("{f:%f,i:%u}\n",f,i);

    *((unsigned int *)&f)=i;
    printf("{f:%f,i:%u}\n",f,i);

    f = 0.0f/0.0f;
    if(f != f)
        printf("f != f,%f,%u\n",f,*((unsigned int *)&f));
    if(f > 0.0f || f == 0.0f || f < 0.0f)
        printf("    f > 0.0f || f == 0.0f || f < 0.0f)\n");
    else
        printf("cool!\n");
    return 0;
}

补充一下c++的nan怎么出

#include <limits>
#include <assert.h>

void foo( double a, double b )
{
    assert( a != b );
}

int main()
{
    typedef std::numeric_limits<double> Info;
    double const nan1 = Info::quiet_NaN();
    double const nan2 = Info::quiet_NaN();
    foo( nan1, nan2 );
}

原来，鱼非鱼啊