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CS253 Exceptions

Overhead

To use the standard exceptions objects, you need to:

#include <stdexcept>

Nomenclature

Sloppy programmers treat the word “throw” as synonymous with “produce an error message”. They might say that make “threw an error”, which is foolish. The make command no more threw an error than it segfaulted. An experienced programmer would, instead, say that make produced error messages, or spat out an error, or errored out, or, simply, failed.

Sure, others will understand you if you use “throw” in this fashion. They’ll also understand you if you speak of “arm tubes” instead of sleeves. However, if you presented yourself as an expert in English, and said “arm tubes”, people wouldn’t believe you. Similarly, using “throw” in that sloppy fashion will cause me to question your competence as a programmer.

Reserve the word “throw” to refer to exceptions, in the throw/try/catch sense. Learn the jargon of your profession.

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 << "power\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

Note that our implementation displayed the type of the thing thrown, but not its contents.

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.

domain_error, invalid_argument, length_error, out_of_range, and future_error all is-a logic_error
range_error, overflow_error, underflow_error, and system_error all is-a runtime_error
logic_error and runtime_error both is-a exception

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 │
        	    └──────────────┘

Of, if you like a diagram:

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!

Yes, literally, catch (...). Dot dot dot. There’s no variable, so the code can’t tell what was thrown.

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 "🐷 🐷 🐷 🐷 🐷 🐷";
}

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?

"🐷 🐷 🐷 🐷 🐷 🐷" is a const char [], or const char *, and we’re catching a string. Those are different types.

catch #2¾

try {
    throw "🐵 🐵 🐵 🐵 🐵 🐵"s;
}

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

Better: 🐵 🐵 🐵 🐵 🐵 🐵

Hey, that worked!

catch #2⅞

try {
    throw "🐼 🐼 🐼 🐼 🐼 🐼"s;
}

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

Even betterer: 🐼 🐼 🐼 🐼 🐼 🐼

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:2: warning: catching polymorphic type ‘class std::exception’ by value
caught: std::exception
caught: 🍩🍩🍩

The base class exception is fast, slim & trim, and has no space for a message. Its virtual .what() method returns a fixed string.
Also, catching an object by value is slow, because the object gets copied, same as passing an object to a function as an argument by value.

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 2022

Exceptions

CS253 Exceptions

Overhead

Nomenclature

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