快速观察给定精度将双精度转换为字符串 [英] Convert double to string quickly observing given precision

问题描述

我有一个使用 SDL 的 C++ 程序.在渲染过程中，我需要绘制一些图形组件.我有时需要将 double 变量(四舍五入到一位小数)转换为 std::string.

I have a C++ program using SDL. During rendering, I need to draw some graphics components. I sometimes need to convert double variables, rounded to one decimal, to std::string.

为此，我目前正在使用 ostringstream 对象并且它工作正常.

For this, I'm currently using a ostringstream object and it works fine.

std::ostringstream ss;
ss << std::fixed << std::setprecision(1) << x;

但是，我想知道这种转换变量的方式在性能方面是否是一个好主意.

However, I'm wondering if this way of converting variables is a good idea with regards to performance.

我试图用 std::to_string(std::round(x * 10)/10)) 对 double 变量进行四舍五入，但它没有t 工作 - 我仍然得到像 2.500000000 这样的输出.

I've tried to round the double variables with std::to_string(std::round(x * 10) / 10)), but it didn't work -- I still got output like 2.500000000.

• 还有其他解决方案吗?
• ostringstream 是否会产生大量成本?

• Is there another solution?
• Does ostringstream incur a heavy cost?

推荐答案

你不能用 std::to_string 指定精度，因为它直接等价于 printf 带参数 %f(如果使用 double).

You can't specify the precision with std::to_string as it is a direct equivalent to printf with the parameter %f (if using double).

如果您担心不会每次都分配流，您可以执行以下操作:

If you are concerned about not allocating each time the stream, you can do the following :

#include <iostream>
#include <sstream>
#include <iomanip>

std::string convertToString(const double & x, const int & precision = 1)
{
    static std::ostringstream ss;
    ss.str(""); // don't forget to empty the stream
    ss << std::fixed << std::setprecision(precision) << x;

    return ss.str();
}


int main() {
    double x = 2.50000;

    std::cout << convertToString(x, 5) << std::endl;
    std::cout << convertToString(x, 1) << std::endl;
    std::cout << convertToString(x, 3) << std::endl;

    return 0;
}

它输出(参见 Coliru):

2.50000

2.5

2.500

虽然我没有检查性能......但我认为你可以通过将它封装到一个类中来做得更好(比如只调用 std::fixed 和 std::精度一次).

I didn't check the performance though... but I think you could even do better by encapsulating this into a class (like only call std::fixed and std::precision once).

否则，您仍然可以使用带有适合您的参数的 sprintf.

Otherwise, you could still use sprintf with the parameters that suits you.

更进一步，使用封装类，您可以将其用作静态实例或另一个类的成员......如您所愿(查看 Coliru).

Going a little further, with an encapsulating class that you could use as a static instance or a member of another class... as you wish (View on Coliru).

#include <iostream>
#include <sstream>
#include <iomanip>

class DoubleToString
{
public:
    DoubleToString(const unsigned int & precision = 1)
    {
        _ss << std::fixed;
        _ss << std::setprecision(precision);
    }

    std::string operator() (const double & x)
    {
        _ss.str("");
        _ss << x;
        return _ss.str();
    }

private:
    std::ostringstream _ss;

};


int main() {
    double x = 2.50000;

    DoubleToString converter;

    std::cout << converter(x) << std::endl;

    return 0;
}

<小时>

另一种不使用 ostringstream 的解决方案(查看 Coliru) :

#include <iostream>
#include <string>
#include <memory>

std::string my_to_string(const double & value) {
  const int length = std::snprintf(nullptr, 0, "%.1f", value);

  std::unique_ptr<char[]> buf(new char[length + 1]);
  std::snprintf(buf.get(), length + 1, "%.1f", value);

  return std::string(buf.get());
}

int main(int argc, char * argv[])
{

    std::cout << my_to_string(argc) << std::endl;
    std::cout << my_to_string(2.5156) << std::endl;

}

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