How to throw a C++ exception

280

I have a very poor understanding of exception handling(i.e., how to customize throw, try, catch statements for my own purposes).

For example, I have defined a function as follows: int compare(int a, int b){...}

I'd like the function to throw an exception with some message when either a or b is negative.

How should I approach this in the definition of the function?

This question is tagged with c++ exception-handling

~ Asked on 2011-12-12 20:51:05

The Best Answer is


394

Simple:

#include <stdexcept>

int compare( int a, int b ) {
    if ( a < 0 || b < 0 ) {
        throw std::invalid_argument( "received negative value" );
    }
}

The Standard Library comes with a nice collection of built-in exception objects you can throw. Keep in mind that you should always throw by value and catch by reference:

try {
    compare( -1, 3 );
}
catch( const std::invalid_argument& e ) {
    // do stuff with exception... 
}

You can have multiple catch() statements after each try, so you can handle different exception types separately if you want.

You can also re-throw exceptions:

catch( const std::invalid_argument& e ) {
    // do something

    // let someone higher up the call stack handle it if they want
    throw;
}

And to catch exceptions regardless of type:

catch( ... ) { };

~ Answered on 2011-12-12 20:53:12


19

Just add throw where needed, and try block to the caller that handles the error. By convention you should only throw things that derive from std::exception, so include <stdexcept> first.

int compare(int a, int b) {
    if (a < 0 || b < 0) {
        throw std::invalid_argument("a or b negative");
    }
}

void foo() {
    try {
        compare(-1, 0);
    } catch (const std::invalid_argument& e) {
        // ...
    }
}

Also, look into Boost.Exception.

~ Answered on 2011-12-12 20:55:47


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