如何在 cpp forloop 中更有效地使用 iostream？

Question

I have some following code. In order to limit call times of operator << to std::cout . I use an std::ostream outside forloop. But I get compile errors.

• source code shows:

#include <iostream>

int square(int num) {

    std::ostream os;
    for (int i=0; i<10; i++) {
        os << "test: i," << std::endl;
    }
    std::cout << os;

    return 0;
}

• error shows:

In file included from /opt/compiler-explorer/gcc-5.5.0/include/c++/5.5.0/iostream:39:0,
                 from <source>:2:
/opt/compiler-explorer/gcc-5.5.0/include/c++/5.5.0/ostream: In function 'int square(int)':
/opt/compiler-explorer/gcc-5.5.0/include/c++/5.5.0/ostream:384:7: error: 'std::basic_ostream<_CharT, _Traits>::basic_ostream() [with _CharT = char; _Traits = std::char_traits<char>]' is protected
       basic_ostream()
       ^
<source>:6:18: error: within this context
     std::ostream os;
                  ^
Compiler returned: 1

Does the smart code with efficiency exist?

Let me make my intension clear.

I don't want to call api and write date to std::cout by multiple times in a loop. I know that I can use a string buffer to receive data multiple times, and finally print to std::cout. But this also takes the cost of creating a string. Is there any directly use of the api or structure provided by <iostream> to write better code?

Answer 1

I'm not sure about efficiency, but what you want to do looks like using std::stringstream .

#include <iostream>
#include <sstream>

int square(int num) {

    std::stringstream ss;
    for (int i=0; i<10; i++) {
        ss << "test: i," << std::endl;
    }
    std::cout << ss.str();

    return 0;
}

Answer 2

If you want efficiency, you there are a few things you could do.

If you want to keep using iostreams, you should take a look at std::ios_base::sync_with_stdio , it can make std::cout much faster, but you wouldn't be able to use C functions to read/write from streams. You also shouldn't use std::endl , as it forces a stream flush, use '\n' instead.

#include <iostream>

int square(int num) {
    for (int i=0; i<10; i++) {
        std::cout << "test: " << i << '\n';
    }
    return 0;
}

int main(){
    std::ios_base::sync_with_stdio(false);
    ...
}

You could also use an external library, fmt is amaizing for that.

#include <fmt/core.h>

int square(int num) {
    for (int i=0; i<10; i++) {
        fmt::print("test: {}\n", i);
    }
    return 0;
}