try block
An exception thrown in a try block can possibly be handled by an associated handler.
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Syntax
try compound-statement handler-seq
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(1) | ||||||||
try ctor-initializer (optional) compound-statement handler-seq
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(2) | ||||||||
| compound-statement | - | a compound statement |
| handler-seq | - | a non-empty sequence of handlers |
| ctor-initializer | - | member initializer list (for constructors only) |
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 }
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.
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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).
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; X() try {} catch (...) { // handles the exception 1 } ~X() 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.
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: ;
Keywords
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 |
