Different meanings of parentheses in C++?

Question

I am a bit confused withnthe interpretation of parentheses by the compiler. Can some one please explain what actually happens in such contexts?

Casting: (int)a

Answer 1

From C++14 Appendix A, the complete list of cases where parentheses may appear in the grammar is:

§A.14 Preprocessing directives
control-line: # define identifier lparen identifier-list_opt ) replacement-list new-line
control-line: # define identifier lparen ... ) replacement-list new-line
control-line: # define identifier lparen identifier-list , ... ) replacement-list new-line

§A.2 Lexical conventions
raw-string: " d-char-sequence_opt ( r-char-sequence_opt ) d-char-sequence_opt "

§A.4 Expressions
primary-expression: ( expression )
lambda-declarator: ( parameter-declaration-clause ) mutable_opt exception-specification_opt attribute-specifier-seq_opt trailing-return-type_opt
postfix-expression: const_cast < type-id > ( expression )
postfix-expression: dynamic_cast < type-id > ( expression )
postfix-expression: postfix-expression ( expression-list_opt )
postfix-expression: reinterpret_cast < type-id > ( expression )
postfix-expression: simple-type-specifier ( expression-list_opt )
postfix-expression: static_cast < type-id > ( expression )
postfix-expression: typeid ( expression )
postfix-expression: typeid ( type-id )
postfix-expression: typename-specifier ( expression-list_opt )
unary-expression: alignof ( type-id )
unary-expression: sizeof ( type-id )
unary-expression: sizeof ... ( identifier )
new-expression: ::_opt new new-placement_opt ( type-id ) new-initializer_opt
new-placement: ( expression-list )
new-initializer: ( expression-list_opt )
noexcept-expression: noexcept ( expression )
cast-expression: ( type-id ) cast-expression

§A.5 Statements
selection-statement: if ( condition ) statement
selection-statement: if ( condition ) statement else statement
selection-statement: switch ( condition ) statement
iteration-statement: do statement while ( expression ) ;
iteration-statement: for ( for-init-statement condition_opt ; expression_opt ) statement
iteration-statement: for ( for-range-declaration : for-range-initializer ) statement
iteration-statement: while ( condition ) statement

§A.6 Declarations
static_assert-declaration: static_assert ( constant-expression , string-literal ) ;
decltype-specifier: decltype ( auto )
decltype-specifier: decltype ( expression )
asm-definition: asm ( string-literal ) ;
alignment-specifier: alignas ( assignment-expression ..._opt )
alignment-specifier: alignas ( type-id ..._opt )
attribute-argument-clause: ( balanced-token-seq )
balanced-token: ( balanced-token-seq )

§A.7 Declarators
noptr-declarator: ( ptr-declarator )
parameters-and-qualifiers: ( parameter-declaration-clause ) attribute-specifier-seq_opt cv-qualifier-seq_opt ref-qualifier_opt exception-specification_opt
noptr-abstract-declarator: ( ptr-abstract-declarator )
initializer: ( expression-list )

§A.10 Special member functions
mem-initializer: mem-initializer-id ( expression-list_opt )

§A.11 Overloading
operator-function-id: operator ( )

§A.13 Exception handling
handler: catch ( exception-declaration ) compound-statement
dynamic-exception-specification: throw ( type-id-list_opt )
noexcept-specification: noexcept ( constant-expression )

Note that:

• The preprocessor rules for if-group and elif-group do refer to constant-expression.
• lparen means a ( with no preceding whitespace
• The rule for raw-string is during lexing, so the ( and ) do not become tokens.
• Any sequence of valid tokens can appear in a preprocessor group whose condition evaluates to false.

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In your question, you use the following:

• cast-expression: ( type-id ) cast-expression
• postfix-expression: simple-type-specifier ( expression-list_opt )
• parameters-and-qualifiers: ( parameter-declaration-clause ) attribute-specifier-seq_opt cv-qualifier-seq_opt ref-qualifier_opt exception-specification_opt
• noptr-abstract-declarator: ( ptr-abstract-declarator )