Difference between revisions of "cpp/language/try catch"

Revision as of 21:57, 19 November 2023

Associates one or more exception handlers (catch-clauses) with a compound statement.

Syntax

where handler-sequence is a sequence of one or more handlers, which have the following syntax:

try { /* */ } catch (const std::exception& e) { /* */ }

try { /* */ } catch (const std::exception&) { /* */ }

try { /* */ } catch (...) { /* */ }

Explanation

See throw exceptions for more information about throw-expressions

A try-block is a statement, and as such, can appear anywhere a statement can appear (that is, as one of the statements in a compound statement, including the function body compound statement). See function-try-block for the try blocks around function bodies. The following description applies to both try-blocks and function-try-blocks.

The formal parameter of the catch clause (type-specifier-seq and declarator or type-specifier-seq and abstract-declarator) determines which types of exceptions cause this catch clause to be entered. It cannot be an incomplete type, abstract class type, rvalue reference type,(since C++11) or pointer to incomplete type (except that pointers to (possibly cv-qualified) void are allowed). If the type of the formal parameter is array type or function type, it is treated as the corresponding pointer type (similar to a function declaration).

When an exception is thrown by any statement in compound-statement, the exception object of type E is matched against the types of the formal parameters T of each catch-clause in handler-seq, in the order in which the catch clauses are listed. The exception is a match if any of the following is true:

• E and T are the same type (ignoring top-level cv-qualifiers on T)
• T is an lvalue-reference to (possibly cv-qualified) E
• T is an unambiguous public base class of E
• T is a reference to an unambiguous public base class of E
• T is (possibly cv-qualified) U or const U&, and U is a pointer or pointer to member type, and E is also a pointer or pointer to member type that is implicitly convertible to U by one or more of

• a standard pointer conversion other than one to a private, protected, or ambiguous base class
• a qualification conversion

try
{
    f();
}
catch (const std::overflow_error& e)
{} // this executes if f() throws std::overflow_error (same type rule)
catch (const std::runtime_error& e)
{} // this executes if f() throws std::underflow_error (base class rule)
catch (const std::exception& e)
{} // this executes if f() throws std::logic_error (base class rule)
catch (...)
{} // this executes if f() throws std::string or int or any other unrelated type

The catch-all clause catch (...) matches exceptions of any type. If present, it has to be the last catch clause in the handler-seq. Catch-all block may be used to ensure that no uncaught exceptions can possibly escape from a function that offers nothrow exception guarantee.

If no matches are found after all catch-clauses were examined, the exception propagation continues to the containing try-block, as described in throw-expression. If there are no containing try-blocks left, std::terminate is executed (in this case, it is implementation-defined whether any stack unwinding occurs at all: throwing an uncaught exception is permitted to terminate the program without invoking any destructors).

When entering a catch clause, if its formal parameter is a base class of the exception type, it is copy-initialized from the base class subobject of the exception object. Otherwise, it is copy-initialized from the exception object (this copy is subject to copy elision).

try
{
    std::string("abc").substr(10); // throws std::length_error
}
// catch (std::exception e) // copy-initialization from the std::exception base
// {
//     std::cout << e.what(); // information from length_error is lost
// }
catch (const std::exception& e) // reference to the base of a polymorphic object
{
    std::cout << e.what(); // information from length_error printed
}

If the parameter of the catch-clause is a reference type, any changes made to it are reflected in the exception object, and can be observed by another handler if the exception is rethrown with throw;. If the parameter is not a reference, any changes made to it are local and its lifetime ends when the handler exits.

A goto or switch statement shall not be used to transfer control into a try block or into a handler.

Other than by throwing or rethrowing the exception, the catch-clause after a regular try block (not function-try-block) may be exited with a return, continue, break, goto, or by reaching the end of its compound-statement. In any case, this destroys the exception object (unless an instance of std::exception_ptr exists that refers to it).

Notes

The throw-expression throw NULL; is not guaranteed to be matched by a pointer catch clause, because the exception object type may be int, but throw nullptr; is assuredly matched by any pointer or pointer-to-member catch clause.

If a catch-clause for a derived class is placed after the catch-clause for a base class, the derived catch-clause will never be executed:

try
{
    f();
}
catch (const std::exception& e)
{} // will be executed if f() throws std::runtime_error
catch (const std::runtime_error& e)
{} // dead code!

If goto is used to exit a try-block and if any of the destructors of block-scoped automatic variables that are executed by the goto throw exceptions, those exceptions are caught by the try blocks in which the variables are defined:

label:
    try
    {
        T1 t1;
        try
        {
            T2 t2;
            if (condition)
                goto label; // destroys t2, then destroys t1, then jumps to label
        }
        catch (...) {} // catches the exception from the destructor of t2
    }
    catch (...) {}     // catches the exception from the destructor of t1

Many implementations overly extend the resolution of CWG388 to handlers of reference of a non-const pointer types:

int i;
try {
    try {
        throw static_cast<float*>(nullptr);
    }
    catch (void*& pv) {
        pv = &i;
        throw;
    }
}
catch (const float* pf) {
    assert(pf == nullptr); // should pass, but fails on mainstream implementations
}

Keywords

try, catch, throw

Example

#include <iostream>
#include <vector>
 
int main()
{
    try
    {
        std::cout << "Throwing an integer exception...\n";
        throw 42;
    }
    catch (int i)
    {
        std::cout << " the integer exception was caught, with value: " << i << '\n';
    }
 
    try
    {
        std::cout << "Creating a vector of size 5... \n";
        std::vector<int> v(5);
        std::cout << "Accessing the 11th element of the vector...\n";
        std::cout << v.at(10); // vector::at() throws std::out_of_range
    }
    catch (const std::exception& e) // caught by reference to base
    {
        std::cout << " a standard exception was caught, with message '"
                  << e.what() << "'\n";
    }
}

Throwing an integer exception...
 the integer exception was caught, with value: 42
Creating a vector of size 5...
Accessing the 11th element of the vector...
 a standard exception was caught, with message 'out_of_range'

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.