FAQ 9.09 When should a function throw an exception?

When it can't fulfill its promises.

When a function detects a problem that prevents it from fulfilling its promises, the function should throw an exception. If the function can recover from the problem so that it can still provide its user with the services it promised, then it has handled the problem and should not throw an exception.

Ideally, exceptions are rare in practice (less than 1% of the time). If an event happens much more frequently than this, perhaps it is not an error, and perhaps the exception-handling mechanism isn't the right choice.

In the following example, the gardener throws an exception if he cannot mow the lawn on the same day the user requests the lawn to be mowed. This occurs when the gardener's lawn mower runs out of gas after 5:00 P.M. (when the gas stations close).

 #include <iostream> #include <cstdlib> #include <stdexcept> #include <string> using namespace std; // pretend this returns the current hour (0 .. 23): int currentHourHand() throw() { return rand() % 24; } class NoGas : public runtime_error { public:   NoGas(const string& what) throw(); }; NoGas::NoGas(const string& what) : runtime_error (what) { } class LawnMower { public:   LawnMower() throw();   void mowOneRow() throw(NoGas);   void fillErUp() throw(NoGas);   bool empty() const throw(); protected:   unsigned gasLeft_; }; LawnMower::LawnMower() throw() : gasLeft_(10) { } void LawnMower::mowOneRow() throw(NoGas) {   if (empty()) fillErUp();   --gasLeft_; } void LawnMower::fillErUp() throw(NoGas) {   if (currentHourHand() >= 17) {     cout << "sorry, gas stations are closed at this hour\n";     throw NoGas("Gas stations are closed");   }   cout << "filling up the tank\n";   gasLeft_ = 10; } bool LawnMower::empty() const throw() { return gasLeft_ == 0; } class Gardener { public:   Gardener() throw();   void mowTheLawn() throw(NoGas); protected:   LawnMower mower_;   int       rowsInLawn_; }; Gardener::Gardener() throw() : mower_() , rowsInLawn_(25) { } void Gardener::mowTheLawn() throw(NoGas) {   for (int row = 0; row < rowsInLawn_; ++row) {     cout << "starting to mow a row\n";     mower_.mowOneRow();   } } int main() {   Gardener mac;   try {     cout << "Mac is trying to mow the lawn\n";     mac.mowTheLawn();     cout << "Mac succeeded at mowing the lawn!\n";   }   catch (NoGas) {     cout << "sorry, Mac ran out of gas after 5pm\n";   } }