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POdifference in two ways of declaring a class instance
 

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    <p>Assume we have a class with no default constructor:</p> <pre><code>class Foo { public: Foo(int data) : _data(data) { } int data(void) const { return _data; } private: int _data; }; </code></pre> <p>Why does this compile and what does it do:</p> <pre><code>Foo x(); </code></pre> <p>Even though the above compiles, you can't do any of the following:</p> <pre><code>x.data(); // doesn't compile: request for member 'data' in 'x', which is of non-class type 'Foo()' x-&gt;data(); // doesn't compile: request for member 'data' in 'x', which is of non-class type 'Foo()' x().data(); // compiles!!! but linking fails with: undefined reference to `x()' x()-&gt;data();// doesn't compile: base operand of '-&gt;' has non-pointer type 'Foo' </code></pre> <p>I guess I'm just confused about what adding the () after the x does, and why the language allows this? Is this every allowable and/or useful? It seems that no Foo instance is allocated on the stack either, because even with -Wunused-variable, no warning occurs about the x in <code>Foo x();</code></p> <p>In contrast, both of the following do not compile:</p> <pre><code>Foo *x = new Foo; // error: no matching function for call to 'Foo::Foo() Foo *y = new Foo();// error: no matching function for call to 'Foo::Foo() </code></pre> <p>This seems more consistent, I don't understand what is going on with <code>Foo x();</code></p> <p>EDIT: Ok, figured it out. <code>Foo x()</code>; is a prototype for a function 'x', which takes no parameters and returns a Foo. Since the function isn't defined anywhere, trying to use it like a Foo instance (<code>x.data()</code>) or Foo pointer (<code>x-&gt;data()</code>) don't work. <code>x()-&gt;data()</code> doesn't work because <code>x()</code> is of type <code>Foo</code>, not pointer to <code>Foo</code>. <code>x().data()</code> compiles because <code>x()</code> returns <code>Foo</code>, which has a <code>data</code> method, but it fails to link because the function <code>x</code> has not been defined. Whew.</p>
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      1. USMahesh
 

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