Direct-initializing an object inside a condition

Question

It is possible to define and copy-initialize a variable inside the condition of an if statement :

if(int i = 17) { ... }

This also works with user-defined types, given that they overload operator bool :

if(Foo f = 42)      { ... }
if(Foo f = Foo(43)) { ... }

Why can't I use direct-initialization, like the following ?

if(Foo f(51)) { ... }

GCC emits error: expected primary-expression before 'f'.

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Is there a reason other than "because the grammar says so" ? And how can I work around it ?

I'm working with VC++03, where Foo :

• is a RAII-sensitive object, for which I took care not to define a copy constructor
• is a template taking arguments from the user
• has a two-parameters constructor

... so I'd rather avoid copying it or repeating its type.

Note : Although my actual problem is with C++03, I'm (academically) interested about answers in C++11.

Answer 1

In C++03, one could solely use the copy-initialization syntax:

selection-statement:
       if ( condition ) statement
        […]

condition:
       expression
       type-specifier-seq declarator = assignment-expression

Since C++11, list-initialization was added:

condition:
        expression
       attribute-specifier-seq_optdecl-specifier-seq declarator = initializer-clause
       attribute-specifier-seq_optdecl-specifier-seq declarator braced-init-list

The syntax of direct-initialization, i.e. Foo f(…), was presumably avoided for the same reason it was disallowed for non-static data member initializers: Ambiguities, in particular the "most vexing parse".

Answer 2

Because the C++03 standard only allows assignment initialisation inside conditions:

condition:
    expression
    type-specifier-seq declarator = assignment-expression