为什么这段代码时不时会产生不同的output？

Question

Compiler:

➜ ~ /usr/bin/c++ --version
Apple clang version 11.0.0 (clang-1100.0.33.8)
Target: x86_64-apple-darwin19.0.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Code:

#include <iostream>

#include <exception>
#include <cstdlib>


void f() {
    try {
        throw std::runtime_error("Exception message.");
    } catch(std::runtime_error& e) {
        std::cout << e.what() << std::endl;
        throw;
    }
}


int main() {
    using namespace std;
    try {
        f();
    } catch(std::exception& e) {
        cout << e.what() << endl;
        terminate();
    }
    return EXIT_SUCCESS;
}

I found out that if i get rid of the "terminate()" in my catch it would work but i am not sure why that is?

There are three outputs that are produced randomly:

libc++abi.dylib: terminating with uncaught exception of type std::runtime_error: Exception message.
Exception message.
Exception message.


Exception message.
libc++abi.dylib: terminating with uncaught exception of type std::runtime_error: Exception message.
Exception message.


Exception message.
Exception message.
libc++abi.dylib: terminating with uncaught exception of type std::runtime_error: Exception message.

PS Sorry, maybe for the incorrect question, I'm a beginner.

Answer 1

For starters, your regular cout print goes to stdout , but the "Terminating with..." message goes to stderr . What you observe stems from the fact that your terminal (or IDE) is reading these two output channels and presenting them in one place. Depending on what algorithm your terminal/IDE uses to merge these two streams, it can happen that the stderr message appears in the places you notice.

If you instead point the streams toward the same file descriptor, your OS will take care of the merging. Run your program as follows:

./my_program 2>&1

Now your terminal will show the messages sequentially with no chance for accidental reordering.