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CS253 Exceptions

`Loud`

These example use my non-standard Loud class.

// A “Loud” class.  It announces whenever its methods are called.
#ifndef LOUD_H_INCLUDED
#define LOUD_H_INCLUDED

#include <iostream>

class Loud {
    char c;
    void hi(const char *s) const {
        std::cout << "Loud::" << s;
        if (c) std::cout << " [c='" << c << "']";
        std::cout << std::endl;  // flush debug output
    }
  public:
    Loud(char ch = '\0') : c(ch) { hi("Loud()"); }
    ~Loud() { hi("~Loud()"); }
    Loud(const Loud &l) : c(l.c) { hi("Loud(const Loud &)"); }
    Loud(Loud &&l) : c(l.c) { hi("Loud(Loud &&)"); }
    Loud& operator=(const Loud &l) { c=l.c; hi("operator=(const Loud &)"); return *this; }
    Loud& operator=(Loud &&l) { c=l.c; hi("operator=(Loud &&)"); return *this; }
    Loud& operator=(char ch) { c = ch; hi("operator=(char)"); return *this; }
    Loud& operator++() { ++c; hi("operator++()"); return *this; }
    Loud operator++(int) { hi("operator++(int)"); const auto save = *this; ++*this; return save; }
    Loud operator+(const Loud &l) const { hi("operator+(const Loud &)"); return Loud(c+l.c); }
};

#endif /* LOUD_H_INCLUDED */

Basic Syntax

try { … }: Execute some code, which might throw an exception.
throw value;: Throw an exception. At that point, program execution transfers to any applicable catch clause. If no catch clause applies, then the program dies.
catch (specification) { … }: Deal with an exception.
specification is very much like a function argument definition.

Normal Operation

void snap() {
    cout << "mind\n";
    throw "Thanos";
    throw "time";       // 💀💀💀
    cout << "space\n";  // 💀💀💀
}

int main() {
    cout << "reality\n";
    try {
        cout << "soul\n";
        snap();
        cout << "gauntlet\n";  // 💀💀💀
    }
    catch (const char *who) {
        cout << "Caught " << who << '\n';
    }
    cout << "50%\n";
}

reality
soul
mind
Caught Thanos
50%

throw without catch

If something is thrown but never caught, then the special function terminate() is called, which complains in an implementation-defined manner, and stops the program by calling abort().

throw "ouch";

terminate called after throwing an instance of 'char const*'
SIGABRT: Aborted

A Special Case

For g++, an uncaught standard exception (derived from std::exception) gets its .what() string displayed.

throw overflow_error("Politician IQ underflow!");

terminate called after throwing an instance of 'std::overflow_error'
  what():  Politician IQ underflow!
SIGABRT: Aborted

This behavior is a GNU extension, so not mandated by the standard, and it sure makes it easier to debug an uncaught exception.

Objects get destroyed appropriately

Loud a('a');

void foo() {
    Loud b('b');
    Loud c('c');
}

int main() {
    Loud d('d');
    foo();
    Loud e('e');
    return 0;
}

Loud::Loud() [c='a']
Loud::Loud() [c='d']
Loud::Loud() [c='b']
Loud::Loud() [c='c']
Loud::~Loud() [c='c']
Loud::~Loud() [c='b']
Loud::Loud() [c='e']
Loud::~Loud() [c='e']
Loud::~Loud() [c='d']
Loud::~Loud() [c='a']

throw without catch

Loud a('a');

void foo() {
    Loud b('b');
    throw 42;
    Loud c('c');
}

int main() {
    Loud d('d');
    foo();
    Loud e('e');
    return 0;
}

Loud::Loud() [c='a']
Loud::Loud() [c='d']
Loud::Loud() [c='b']
terminate called after throwing an instance of 'int'
SIGABRT: Aborted

You can throw anything—doesn’t have to be a special type or object.

I mean anything

Really—you can throw and catch any type. For example:

throw 42;

terminate called after throwing an instance of 'int'
SIGABRT: Aborted

throw "alpha";

terminate called after throwing an instance of 'char const*'
SIGABRT: Aborted

throw "beta"s;

terminate called after throwing an instance of 'std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >'
SIGABRT: Aborted

I mean anything

int n=42; throw "Value no good: " + to_string(n);

terminate called after throwing an instance of 'std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >'
SIGABRT: Aborted

throw logic_error("That failed miserably");

terminate called after throwing an instance of 'std::logic_error'
  what():  That failed miserably
SIGABRT: Aborted

It’s nice that the what() value for a std::exception gets displayed.

Standard Exceptions

There are a number of classes defined by the C++ standard, in <stdexcept>. It’s best to use them, or classes derived from them, as opposed to rolling your own.

Standard Exception Class Hierarchy

        	      ┌─────────────────────────┐
        	      │	       exception	│
        	      └─────────────────────────┘
        		△		      △
        		│		      │
            ┌───────────┴──────────┐	  ┌───┴───────────────┐
            │	   logic_error	   │	  │   runtime_error   │
            └──────────────────────┘	  └───────────────────┘
             △	     △	 △   △	 △	   △	    △	△   △
             │	     │	 │   │	 │	   │	    │	│   │
    ┌────────┴─────┐ │	 │   │	 │ ┌───────┴──────┐ │	│   │
    │ domain_error │ │	 │   │	 │ │ range_error  │ │	│   │
    └──────────────┘ │	 │   │	 │ └──────────────┘ │	│   │
    ┌────────────────┴─┐ │   │	 │   ┌──────────────┴─┐ │   │
    │ invalid_argument │ │   │	 │   │ overflow_error │ │   │
    └──────────────────┘ │   │	 │   └────────────────┘ │   │
            ┌────────────┴─┐ │	 │	┌───────────────┴─┐ │
            │ length_error │ │	 │	│ underflow_error │ │
            └──────────────┘ │	 │	└─────────────────┘ │
        	┌────────────┴─┐ │	       ┌────────────┴─┐
        	│ out_of_range │ │	       │ system_error │
        	└──────────────┘ │	       └──────────────┘
        	    ┌────────────┴─┐
        	    │ future_error │
        	    └──────────────┘

try #1

Loud a('a');

void foo() {
    Loud b('b');
    throw "oops!";
    Loud c('c');
}

int main() {
    Loud d('d');
    try {
        foo();
    }
    catch (const char *error) {
        cout << "Caught: " << error << "\n";
    }

    Loud e('e');
    return 0;
}

Loud::Loud() [c='a']
Loud::Loud() [c='d']
Loud::Loud() [c='b']
Loud::~Loud() [c='b']
Caught: oops!
Loud::Loud() [c='e']
Loud::~Loud() [c='e']
Loud::~Loud() [c='d']
Loud::~Loud() [c='a']

try #2

Loud a('a');

void foo() {
    Loud b('b');
    throw "oops!";
    Loud c('c');
}

void bar() {
    Loud d('d');
    foo();
}

int main() {
    Loud e('e');
    try {
        bar();
    }
    catch (const char *error) {
        cout << "Caught: “" << error << "”\n";
    }
    Loud f('f');
}

Loud::Loud() [c='a']
Loud::Loud() [c='e']
Loud::Loud() [c='d']
Loud::Loud() [c='b']
Loud::~Loud() [c='b']
Loud::~Loud() [c='d']
Caught: “oops!”
Loud::Loud() [c='f']
Loud::~Loud() [c='f']
Loud::~Loud() [c='e']
Loud::~Loud() [c='a']

catch #1

try {
    throw "oops!";
}

catch (int i) {
    cout << "int " << i << "\n";
}

catch (const char *error) {
    cout << "C string: " << error << "\n";
}

catch (...) {  // Gotta catch ’em all!
    cout << "something\n";
}

C string: oops!

Multiple catches

Multiple catch clauses are executed in order written.
Therefore, catch the most specific things first, and the least specific things last.
catch (...) is the least specific of all, so it must go last.
A good compiler may warn of an unreachable catch; don’t count on it.

try { }
catch (...) { }
catch (int) { }

c.cc:2: error: '...' handler must be the last handler for its try block

catch #2

try {
    throw 42;
}

catch (short s) {
    cout << "Got a short: " << s << "\n";
}

catch (long l) {
    cout << "Got a long: " << l << "\n";
}

terminate called after throwing an instance of 'int'
SIGABRT: Aborted

The type must match. No conversions! (almost)

catch #2½

try {
    throw "Son of a gun!";
}

catch (string s) {
    cout << "Got a string: " << s << "\n";
}

terminate called after throwing an instance of 'char const*'
SIGABRT: Aborted

Why didn’t that work?

catch #2¾

try {
    throw "That’s better."s;
}

catch (string s) {
    cout << "Got a string: " << s << "\n";
}

Got a string: That’s better.

Hey, that worked!

catch #2⅞

try {
    throw "Even better."s;
}

catch (const string &s) {
    cout << "Got a string: " << s << "\n";
}

Got a string: Even better.

This works, and is more efficient. Of course, efficiency may not be an issue here.

catch #3

// logic_error is-a exception
// runtime_error is-a exception
// overflow_error is-a runtime_error

try {
    throw overflow_error("Just too big!");
}
catch (const logic_error &e) {
    cout << "logic_error: " << e.what() << '\n';
}
catch (const runtime_error &e) {
    cout << "runtime_error: " << e.what() << '\n';
}
catch (const exception &e) {
    cout << "exception: " << e.what() << '\n';
}

runtime_error: Just too big!

The types must match, except that is-a is good enough.

catch #3½

Avoid catching a polymorphic type by value, because of object slicing:

try {
    throw overflow_error("🍕🍕🍕");
}
catch (exception e) {  // ☠ ☠ ☠
    cout << "caught: " << e.what() << '\n';
}

try {
    throw overflow_error("🍩🍩🍩");
}
catch (const exception &f) {
    cout << "caught: " << f.what() << '\n';
}

c.cc:4: warning: catching polymorphic type 'class std::exception' by value
caught: std::exception
caught: 🍩🍩🍩

catch #4

// logic_error is-a exception
// runtime_error is-a exception
// overflow_error is-a runtime_error

try {
    throw overflow_error("Just too big!");
}
catch (const logic_error &e) {
    cout << "logic_error: " << e.what() << '\n';
}
catch (const exception &e) {
    cout << "exception: " << e.what() << '\n';
}

exception: Just too big!

No conversions, but is-a is good enough.

re-throw

void foo() {
    throw "Division by zero"s;
}

void bar() {
    try {
        foo();
    }
    catch (string msg) {
        if (msg == "Out of memory")  // I’ve got this!
            /* get more memory */;
        else
            throw;      // Throw up hands in despair.
    }
}

int main() {
    try {
        bar();
    }
    catch (string problem) {
        cout << "Problem in bar: " << problem << '\n';
    }

    return 0;
}

Problem in bar: Division by zero

Not a panacea

Exceptions are not a panacea, because not all problems cause exceptions.

The vector::at() and string::at() methods are defined to throw a specific exception, out_of_range:

vector<int> v = {11,22,33};
cout << v.at(1000000);

terminate called after throwing an instance of 'std::out_of_range'
  what():  vector::_M_range_check: __n (which is 1000000) >= this->size() (which is 3)
SIGABRT: Aborted

Catching

The exception thrown by vector::at() can be caught:

vector<int> v = {11,22,33};
try {
    cout << v.at(1000000);
}
catch (const exception &e) {
    cerr << "OOPS!  " << e.what() << '\n';
}

OOPS!  vector::_M_range_check: __n (which is 1000000) >= this->size() (which is 3)

Uncatchable

However, the problems caused by vector::operator[] can’t be caught, because they are not defined to throw exceptions:

vector<int> v = {11,22,33};
cout << v[1000000];

SIGSEGV: Segmentation fault

vector::at() runs (slightly) slowly, clutters up your code, and catches your mistakes.
[] runs quickly, is terse, and doesn’t help you when you screw up.

Your choice!

CS253: Software Development with C++

Fall 2020

Exceptions

CS253 Exceptions

`Loud`

Basic Syntax

Normal Operation

throw without catch

A Special Case

Objects get destroyed appropriately

throw without catch

I mean anything

I mean anything

Standard Exceptions

Standard Exception Class Hierarchy

try #1

try #2

catch #1

Multiple catches

catch #2

catch #2½

catch #2¾

catch #2⅞

catch #3

catch #3½

catch #4

re-throw

Not a panacea

Catching

Uncatchable