Difference between revisions of "cpp/language/try"

Latest revision as of 01:55, 26 September 2024

An exception thrown in a try block can possibly be handled by an associated handler.

[edit] Syntax

[edit] Ordinary try block

An ordinary try block is a statement.

If an exception is thrown from its compound-statement, the exception will be matched against the handlers in its handler-seq :

void f()
{
    throw 1;     // NOT handled by the handler below
    try
    {
        throw 2; // handled by the associated handler
    }
    catch (...)
    {
        // handles the exception 2
    }
    throw 3;     // NOT handled by the handler above
}

[edit] Function try block

A function try block is a special kind of function body.

If an exception is thrown from its compound-statement or ctor-initializer (if any), the exception will be matched against the handlers in its handler-seq :

int f(bool cond)
{
    if (cond)
        throw 1;
    return 0;
}
 
struct X
{
    int mem;
 
    X() try : mem(f(true)) {}
    catch (...)
    {
        // handles the exception 1
    }
 
    X(int) try
    {
        throw 2;
    }
    catch (...)
    {
        // handles the exception 2
    }
};

Exceptions thrown in destructors of objects with static storage duration or in constructors of objects associated with non-block variables with static storage duration are not caught by a function try block on the main function.

Flowing off the end of the compound-statement of a handler of a function try block is equivalent to flowing off the end of the compound-statement of that function try block, unless the function is a constructor or destructor (see below).

[edit] Constructor and destructor try block

For a class C, if the function body of its constuctor or destructor definition is a function try block, and an exception is thrown during the initialization or destruction, respectively, of C’s subobjects, the exception will also be matched against the handlers in the handler-seq  of the function try block:

int f(bool cond = true)
{
    if (cond)
        throw 1;
    return 0;
}
 
struct X
{
    int mem = f();
 
    ~X()
    {
        throw 2;
    }
};
 
struct Y
{
    X mem;
 
    Y() try {}
    catch (...)
    {
        // handles the exception 1
    }
 
    ~Y() try {}
    catch (...)
    {
        // handles the exception 2
    }
};

Referring to any non-static member or base class of an object in the handler for a function try block of a constructor or destructor for that object results in undefined behavior.

If a return statement appears in a handler of the function try block of a constructor, the program is ill-formed.

The currently handled exception is rethrown if control reaches the end of a handler of the function try block of a constructor or destructor.

[edit] Control flow

The compound-statement of a try block is a control-flow-limited statement:

void f()
{
    goto label;     // error
    try
    {
        goto label; // OK
        label: ;
    }
    catch (...)
    {
        goto label; // error
    }
}

A jump statement (goto, break, return, continue) can be used to transfer control out of a try block (including its handlers). When this happens, each variable declared in the try block will be destroyed in the context that directly contains its declaration:

try
{
    T1 t1;
    try
    {
        T2 t2;
        goto label; // destroy t2 first, then t1
    }
    catch(...)
    {
        // executed if an exception is thrown while destroying t2
    }
}
catch(...)
{
    // executed if an exception is thrown while destroying t1
}
label: ;

[edit] Keywords

try

[edit] Defect reports

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

[edit] See also

• Throwing exceptions
• Handling exceptions