try block
An exception thrown in a try block can possibly be handled by an associated handler.
Contents |
[edit]Syntax
trycompound-statementhandler-seq | (1) | ||||||||
tryctor-initializer (optional)compound-statementhandler-seq | (2) | ||||||||
| compound-statement | - | a compound statement |
| handler-seq | - | a non-empty sequence of handlers |
| ctor-initializer | - | member initializer list (for constructors only) |
[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(){throw1;// NOT handled by the handler belowtry{throw2;// handled by the associated handler}catch(...){// handles the exception 2}throw3;// 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)throw1;return0;} struct X {int mem; X()try: mem(f(true)){}catch(...){// handles the exception 1} X(int)try{throw2;}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.
Exceptions thrown in destructors of objects with thread storage duration or in constructors of objects associated with non-block variables with thread storage duration are not caught by a function try block on the initial function of the thread. | (since C++11) |
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)throw1;return0;} struct X {int mem = f(); ~X(){throw2;}}; 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;// errortry{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
[edit]Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| CWG 98 | C++98 | a switch statement can transfer control into the compound-statement of a try block | prohibited |
| CWG 1167 | C++98 | it was unspecified whether a function try block on a destructor will catch exceptions from a base or member destructor | such exceptions are caught |
