It is possible to devise syntax that can ambiguously be interpreted as either an expression statement or a declaration. Syntax of this sort is called a vexing parse because the compiler must use disambiguation rules to determine the semantic results. The C++ Standard, [stmt.ambig], paragraph 1 , states in part [ISO/IEC 14882-2014], states in part:
There is an ambiguity in the grammar involving expression-statements and declarations: An expression-statement with a function-style explicit type conversion as its leftmost subexpression can be indistinguishable from a declaration where the first declarator starts with a
(. In those cases the statement is a declaration. [Note: To disambiguate, the whole statement might have to be examined to determine if it is an expression-statement or a declaration. ...
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In this noncompliant code example, an attempt is made to declare a local variable, w, of type Widget while executing the default constructor. However, this declaration is syntactically ambiguous where the code could be either a declaration of a function pointer accepting no arguments and returning a Widget or a declaration of a local variable of type Widget. The syntax used in this example defines the former instead of the latter.
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Running this program produces the expected output:
Widget constructedGadget constructedWidget constructedGadget constructed
3
Risk Assessment
Syntactically ambiguous declarations can lead to unexpected program execution. However, it is likely that rudimentary testing would uncover violations of this rule.
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Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| 2510 | ||||||||
| Clang |
| -Wvexing-parse |
Related Vulnerabilities
Search for other vulnerabilities resulting from the violation of this rule on the CERT website.
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