Boost提供了一组简单的舍入函数。

#include <boost/math/special_functions/round.hpp>

double a = boost::math::round(1.5); // Yields 2.0
int b = boost::math::iround(1.5); // Yields 2 as an integer

有关更多信息，请参阅Boost文档。

编辑:从c++ 11开始，有std::round, std::lround和std::llround。

如果您需要能够在支持c++ 11标准的环境中编译代码，但也需要能够在不支持c++ 11标准的环境中编译相同的代码，那么您可以使用函数宏在std::round()和每个系统的自定义函数之间进行选择。只需将-DCPP11或/DCPP11传递给兼容c++ 11的编译器(或使用其内置的版本宏)，并创建一个像这样的头文件:

// File: rounding.h
#include <cmath>

#ifdef CPP11
    #define ROUND(x) std::round(x)
#else    /* CPP11 */
    inline double myRound(double x) {
        return (x >= 0.0 ? std::floor(x + 0.5) : std::ceil(x - 0.5));
    }

    #define ROUND(x) myRound(x)
#endif   /* CPP11 */

有关快速示例，请参见http://ideone.com/zal709。

这在不兼容c++ 11的环境中近似于std::round()，包括保留-0.0的符号位。然而，这可能会导致轻微的性能损失，并且在舍入某些已知的“问题”浮点值(如0.4999999999999999994或类似值)时可能会出现问题。

或者，如果你有c++ 11兼容的编译器，你可以从它的<cmath>头文件中获取std::round()，并使用它来创建你自己的头文件来定义函数(如果它还没有定义的话)。但是请注意，这可能不是最佳解决方案，特别是如果您需要为多个平台编译时。

将浮点值四舍五入小数“n”位的最佳方法如下，在O(1)时间内:-

我们必须将值四舍五入3位，即n=3。所以,

float a=47.8732355;
printf("%.3f",a);

Boost提供了一组简单的舍入函数。

#include <boost/math/special_functions/round.hpp>

double a = boost::math::round(1.5); // Yields 2.0
int b = boost::math::iround(1.5); // Yields 2 as an integer

有关更多信息，请参阅Boost文档。

编辑:从c++ 11开始，有std::round, std::lround和std::llround。

值得注意的是，如果想要从舍入中得到整数结果，则不需要通过上下限或上下限。也就是说,

int round_int( double r ) {
    return (r > 0.0) ? (r + 0.5) : (r - 0.5); 
}

现在，使用包含C99/ c++ 11数学库的c++ 11编译器应该不是问题。但接下来的问题是:选择哪个舍入函数?

C99/ c++ 11 round()通常不是你想要的舍入函数。它使用了一种时髦的舍入模式，在一半的情况下(+-xxx.5000)舍入0作为抢七。如果你确实特别想要这种舍入模式，或者你的目标是一个round()比rint()更快的c++实现，那么就使用它(或者用这个问题的其他答案之一来模仿它的行为，从表面上看，仔细地复制特定的舍入行为)。

round()的舍入不同于IEEE754默认的舍入到最接近的模式，以偶数作为抢七。最接近偶数避免了数字平均大小的统计偏差，但确实偏向偶数。

有两个数学库舍入函数使用当前默认的舍入模式:std::nearbyint()和std::rint()，它们都是在C99/ c++ 11中添加的，所以它们在std::round()存在的任何时候都可用。唯一的区别是nearbyint从不引发FE_INEXACT。

出于性能考虑，更倾向于rint(): gcc和clang都更容易内联它，但gcc从不内联nearbyint()(即使使用- fast-math)

gcc/clang用于x86-64和AArch64

我把一些测试函数放在Matt Godbolt的编译器资源管理器上，在那里你可以看到source + asm输出(用于多个编译器)。有关阅读编译器输出的更多信息，请参阅此问答和Matt的CppCon2017演讲:“我的编译器最近为我做了什么?”打开编译器的盖子”，

In FP code, it's usually a big win to inline small functions. Especially on non-Windows, where the standard calling convention has no call-preserved registers, so the compiler can't keep any FP values in XMM registers across a call. So even if you don't really know asm, you can still easily see whether it's just a tail-call to the library function or whether it inlined to one or two math instructions. Anything that inlines to one or two instructions is better than a function call (for this particular task on x86 or ARM).

在x86上，任何内联到SSE4.1 roundsd的东西都可以使用SSE4.1 roundpd(或AVX vroundpd)自动向量化。(FP->整数转换也可用打包SIMD形式，除了FP->64位整数，它需要AVX512。)

std::nearbyint(): x86 clang: inlines to a single insn with -msse4.1. x86 gcc: inlines to a single insn only with -msse4.1 -ffast-math, and only on gcc 5.4 and earlier. Later gcc never inlines it (maybe they didn't realize that one of the immediate bits can suppress the inexact exception? That's what clang uses, but older gcc uses the same immediate as for rint when it does inline it) AArch64 gcc6.3: inlines to a single insn by default. std::rint: x86 clang: inlines to a single insn with -msse4.1 x86 gcc7: inlines to a single insn with -msse4.1. (Without SSE4.1, inlines to several instructions) x86 gcc6.x and earlier: inlines to a single insn with -ffast-math -msse4.1. AArch64 gcc: inlines to a single insn by default std::round: x86 clang: doesn't inline x86 gcc: inlines to multiple instructions with -ffast-math -msse4.1, requiring two vector constants. AArch64 gcc: inlines to a single instruction (HW support for this rounding mode as well as IEEE default and most others.) std::floor / std::ceil / std::trunc x86 clang: inlines to a single insn with -msse4.1 x86 gcc7.x: inlines to a single insn with -msse4.1 x86 gcc6.x and earlier: inlines to a single insn with -ffast-math -msse4.1 AArch64 gcc: inlines by default to a single instruction

舍入到int / long / long:

你有两个选择:使用lrint(像rint一样，但返回long，或llrint返回long long)，或使用FP->FP四舍五入函数，然后以正常的方式(带截断)转换为整数类型。有些编译器的一种优化方式比另一种更好。

long l = lrint(x);

int  i = (int)rint(x);

注意int i = lrint(x)首先转换float或double -> long，然后将整型截断为int。对于超出范围的整数，这是有区别的:在c++中未定义行为，但在x86 FP -> int指令中定义良好(编译器将发出除非它在编译时看到UB，同时进行常量传播，那么它被允许使代码在执行时中断)。

On x86, an FP->integer conversion that overflows the integer produces INT_MIN or LLONG_MIN (a bit-pattern of 0x8000000 or the 64-bit equivalent, with just the sign-bit set). Intel calls this the "integer indefinite" value. (See the cvttsd2si manual entry, the SSE2 instruction that converts (with truncation) scalar double to signed integer. It's available with 32-bit or 64-bit integer destination (in 64-bit mode only). There's also a cvtsd2si (convert with current rounding mode), which is what we'd like the compiler to emit, but unfortunately gcc and clang won't do that without -ffast-math.

还要注意，从unsigned int / long到/从unsigned int / long的FP在x86上效率较低(没有AVX512)。在64位机器上转换为32位无符号是非常便宜的;只需转换为64位符号并截断即可。但除此之外，它明显变慢了。

x86 clang with/without -ffast-math -msse4.1: (int/long)rint inlines to roundsd / cvttsd2si. (missed optimization to cvtsd2si). lrint doesn't inline at all. x86 gcc6.x and earlier without -ffast-math: neither way inlines x86 gcc7 without -ffast-math: (int/long)rint rounds and converts separately (with 2 total instructions of SSE4.1 is enabled, otherwise with a bunch of code inlined for rint without roundsd). lrint doesn't inline. x86 gcc with -ffast-math: all ways inline to cvtsd2si (optimal), no need for SSE4.1. AArch64 gcc6.3 without -ffast-math: (int/long)rint inlines to 2 instructions. lrint doesn't inline AArch64 gcc6.3 with -ffast-math: (int/long)rint compiles to a call to lrint. lrint doesn't inline. This may be a missed optimization unless the two instructions we get without -ffast-math are very slow.