2005年提出了技术报告1。TR1为c++带来了与C99的兼容性。尽管它从未被正式采用成为c++标准，但许多(GCC 4.0 +或Visual c++ 9.0+ c++实现确实提供了TR1特性，全部或仅部分(Visual c++ 9.0不提供C99数学函数)。
如果TR1可用，则cmath包含C99元素，如isnan()， isfinite()等，但它们被定义为函数，而不是宏，通常在std::tr1::命名空间中，尽管许多实现(例如Linux上的GCC 4+或Mac OS X 10.5+上的XCode)将它们直接注入std::，因此std::isnan定义良好。
#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 );
}
使用g++ (TDM-2 mingw32) 4.4.1编译:
C:\test> type "C:\Program Files\@commands\gnuc.bat"
@rem -finput-charset=windows-1252
@g++ -O -pedantic -std=c++98 -Wall -Wwrite-strings %* -Wno-long-long
C:\test> gnuc x.cpp
C:\test> a && echo works... || echo !failed
works...
C:\test> gnuc x.cpp --fast-math
C:\test> a && echo works... || echo !failed
Assertion failed: a != b, file x.cpp, line 6
This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
!failed
C:\test> _
#include <float.h>
float x=0.f ; // I'm gonna divide by x!
if( !x ) // Wait! Let me check if x is 0
x = FLT_MIN ; // oh, since x was 0, i'll just make it really small instead.
float y = 0.f / x ; // whew, `nan` didn't appear.
#include <stdint.h>
#include <stdio.h>
union NaN
{
uint64_t bits;
double num;
};
int main()
{
//Test if a double is NaN
double d = 0.0 / 0.0;
union NaN n;
n.num = d;
if((n.bits | 0x800FFFFFFFFFFFFF) == 0xFFFFFFFFFFFFFFFF)
{
printf("NaN: %f", d);
}
return 0;
}
