tmp/tmpinsh6qqx/{from.md → to.md}
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| 1 |
+
## Preamble <a id="except.pre">[[except.pre]]</a>
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| 2 |
+
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| 3 |
+
Exception handling provides a way of transferring control and
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| 4 |
+
information from a point in the execution of a thread to an exception
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| 5 |
+
handler associated with a point previously passed by the execution. A
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+
handler will be invoked only by throwing an exception in code executed
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in the handler’s try block or in functions called from the handler’s try
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block.
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+
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| 10 |
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``` bnf
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| 11 |
+
try-block:
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| 12 |
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'try' compound-statement handler-seq
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| 13 |
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```
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| 14 |
+
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| 15 |
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``` bnf
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| 16 |
+
function-try-block:
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| 17 |
+
'try' ctor-initializerₒₚₜ compound-statement handler-seq
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| 18 |
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```
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| 19 |
+
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| 20 |
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``` bnf
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| 21 |
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handler-seq:
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| 22 |
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handler handler-seqₒₚₜ
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| 23 |
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```
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| 24 |
+
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| 25 |
+
``` bnf
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| 26 |
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handler:
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| 27 |
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'catch' '(' exception-declaration ')' compound-statement
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| 28 |
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```
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``` bnf
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| 31 |
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exception-declaration:
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| 32 |
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attribute-specifier-seqₒₚₜ type-specifier-seq declarator
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| 33 |
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attribute-specifier-seqₒₚₜ type-specifier-seq abstract-declaratorₒₚₜ
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| 34 |
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'...'
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| 35 |
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```
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| 36 |
+
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The optional *attribute-specifier-seq* in an *exception-declaration*
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appertains to the parameter of the catch clause [[except.handle]].
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A *try-block* is a *statement* [[stmt.pre]].
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[*Note 1*: Within this Clause “try block” is taken to mean both
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| 43 |
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*try-block* and *function-try-block*. — *end note*]
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| 44 |
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A `goto` or `switch` statement shall not be used to transfer control
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| 46 |
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into a try block or into a handler.
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| 47 |
+
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| 48 |
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[*Example 1*:
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| 49 |
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| 50 |
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``` cpp
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| 51 |
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void f() {
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| 52 |
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goto l1; // error
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| 53 |
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goto l2; // error
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| 54 |
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try {
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| 55 |
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goto l1; // OK
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| 56 |
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goto l2; // error
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| 57 |
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l1: ;
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| 58 |
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} catch (...) {
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| 59 |
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l2: ;
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| 60 |
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goto l1; // error
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| 61 |
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goto l2; // OK
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| 62 |
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}
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| 63 |
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}
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| 64 |
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```
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| 65 |
+
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— *end example*]
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| 67 |
+
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| 68 |
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A `goto`, `break`, `return`, or `continue` statement can be used to
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| 69 |
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transfer control out of a try block or handler. When this happens, each
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| 70 |
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variable declared in the try block will be destroyed in the context that
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| 71 |
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directly contains its declaration.
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| 72 |
+
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| 73 |
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[*Example 2*:
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| 74 |
+
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| 75 |
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``` cpp
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| 76 |
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lab: try {
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| 77 |
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T1 t1;
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| 78 |
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try {
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| 79 |
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T2 t2;
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| 80 |
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if (condition)
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| 81 |
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goto lab;
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| 82 |
+
} catch(...) { /* handler 2 */ }
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| 83 |
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} catch(...) { /* handler 1 */ }
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| 84 |
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```
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| 85 |
+
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| 86 |
+
Here, executing `goto lab;` will destroy first `t2`, then `t1`, assuming
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| 87 |
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the *condition* does not declare a variable. Any exception thrown while
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| 88 |
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destroying `t2` will result in executing `handler 2`; any exception
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| 89 |
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thrown while destroying `t1` will result in executing `handler 1`.
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| 90 |
+
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| 91 |
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— *end example*]
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| 92 |
+
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| 93 |
+
A *function-try-block* associates a *handler-seq* with the
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| 94 |
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*ctor-initializer*, if present, and the *compound-statement*. An
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| 95 |
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exception thrown during the execution of the *compound-statement* or,
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| 96 |
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for constructors and destructors, during the initialization or
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| 97 |
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destruction, respectively, of the class’s subobjects, transfers control
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| 98 |
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to a handler in a *function-try-block* in the same way as an exception
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| 99 |
+
thrown during the execution of a *try-block* transfers control to other
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| 100 |
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handlers.
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| 101 |
+
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| 102 |
+
[*Example 3*:
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| 103 |
+
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| 104 |
+
``` cpp
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| 105 |
+
int f(int);
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| 106 |
+
class C {
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| 107 |
+
int i;
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| 108 |
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double d;
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| 109 |
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public:
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| 110 |
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C(int, double);
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| 111 |
+
};
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| 112 |
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| 113 |
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C::C(int ii, double id)
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| 114 |
+
try : i(f(ii)), d(id) {
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| 115 |
+
// constructor statements
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| 116 |
+
} catch (...) {
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| 117 |
+
// handles exceptions thrown from the ctor-initializer and from the constructor statements
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| 118 |
+
}
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| 119 |
+
```
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| 120 |
+
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| 121 |
+
— *end example*]
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| 122 |
+
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| 123 |
+
In this Clause, “before” and “after” refer to the “sequenced before”
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| 124 |
+
relation [[intro.execution]].
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| 125 |
+
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