从std :: cout或std :: ofstream（file）获取std :: ostream

问题描述

如何将 std :: ostream 绑定到 std :: cout 或 std :: ofstream 对象，取决于一定的程序条件？虽然这个无效的原因有很多，我想实现在语义上等同于下面的内容：

how do I bind a std::ostream to either std::cout or to an std::ofstream object, depending on a certain program condition? Although this invalid for many reasons, I would like to achieve something that is semantically equivalent to the following:

std::ostream out = condition ? &std::cout : std::ofstream(filename);

我看过一些不是异常安全的例子，例如：

I've seen some examples that are not exception-safe, such as one from http://www2.roguewave.com/support/docs/sourcepro/edition9/html/stdlibug/34-2.html:

int main(int argc, char *argv[])
{
  std::ostream* fp;                                           //1
  if (argc > 1)
     fp = new std::ofstream(argv[1]);                         //2
  else
     fp = &std::cout                                          //3

  *fp << "Hello world!" << std::endl;                         //4
  if (fp!=&std::cout)
     delete fp;
}

有没有人知道更好的异常安全解决方案？

Does anyone know a better, exception-safe solution?

推荐答案

std::streambuf * buf;
std::ofstream of;

if(!condition) {
    of.open("file.txt");
    buf = of.rdbuf();
} else {
    buf = std::cout.rdbuf();
}

std::ostream out(buf);

将cout或输出文件流的底层streambuf关联到out。之后，你可以写到出，它会结束在正确的目的地。如果你只是想让 std :: cout 进入一个文件，你也可以

That associates the underlying streambuf of either cout or the output file stream to out. After that you can write to "out" and it will end up in the right destination. If you just want that everything going to std::cout goes into a file, you can aswell do

std::ofstream file("file.txt");
std::streambuf * old = std::cout.rdbuf(file.rdbuf());
// do here output to std::cout
std::cout.rdbuf(old); // restore

这第二种方法的缺点是它不是异常安全的。你可能想写一个类，使用RAII：

This second method has the drawback that it's not exception safe. You possibly want to write a class that does this using RAII:

struct opiped {
    opiped(std::streambuf * buf, std::ostream & os)
    :os(os), old_buf(os.rdbuf(buf)) { }
    ~opiped() { os.rdbuf(old_buf); }

    std::ostream& os;
    std::streambuf * old_buf;
};

int main() {
    // or: std::filebuf of;
    //     of.open("file.txt", std::ios_base::out);
    std::ofstream of("file.txt");
    {
        // or: opiped raii(&of, std::cout);
        opiped raii(of.rdbuf(), std::cout);
        std::cout << "going into file" << std::endl;
    }
    std::cout << "going on screen" << std::endl;
}

现在，无论发生什么，std :: cout都处于clean状态。

Now, whatever happens, std::cout is in clean state.

这篇关于从std :: cout或std :: ofstream（file）获取std :: ostream的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！