Question

I am a bit puzzled about this. When default stack allocated object construction is allowed as member varaiable of other struct, why not parameterized construction ? Does Most Vexing Parse has anything to do with this ? I tried on C++0X as well at ideone and got the same result.

struct foo{
    foo() {}
    foo(int i) {}
};

struct bar{
  foo obj;      // Allowed
  foo obj2(10); // Not Allowed
};

Error: expected ‘,’ or ‘…’ before numeric constant

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Answer 1

Your link to Most Vexing Parse discusses instantiation of non-member variables (variables which are not a member of a class/struct). The example you’ve shown is that of member variables being declared in a struct; for which you would typically initialise them in a constructor

struct bar
{
    foo obj;
    foo obj2;
    bar() : obj2(10) {}
};

The Most Vexing Parse problem occurs in situations such as the below:

struct baz
{
    baz(int n) {}
};

void foo()
{
    baz meow(int());
}

where the identifier meow appears to be a function declaration of type baz (int), due to the ‘most vexing parse’ issues described in your link. (the int() which at first glance appears to be default-initialisation actually turns out to be simply the data type int)