Why can't i construct a vector by passing temporary input iterator?

Question

I often used istream_iterator copy standard input to a vector like this:

copy(istream_iterator<int>(cin), istream_iterator<int>(), back_inserter(vec));

it works.

Today when i construct a vector by this:

vector<int> vec(istream_iterator<int>(cin), istream_iterator<int>());
copy(vec.begin(), vec.end(), ostream_iterator<int>(cout, " "));

it just can not compile!

But if i construct it like this:

istream_iterator<int> beg(cin), end;
vector<int> vec(beg, end); 
copy(vec.begin(), vec.end(), ostream_iterator<int>(cout, " "));

it works.

Why? Can not i use temporary variables to construct a vector? If it does, but why could i do it when i use std::copy?

ps: I compiled it under vs2005 and i used the following head files:

#include <iostream>
#include <vector>
#include <iterator>
#include <algorithm>

Answer 1

Unfortunately, this is a function declaration:

vector<int> vec(istream_iterator<int>(cin), istream_iterator<int>());

It's a function named vec, that returns a vector<int> by value and takes two parameters: an istream_iterator<int> with a formal parameter name of cin, and a function with no formal parameter name that returns an istream_iterator<int>, and takes no parameters.

Why?

Basically, in C++(and C), if a piece of code can be interpreted as a declaration, it will be.

According to N3936::6.8.1 [stmt.ambig]:

There is an ambiguity in the grammar involving expression-statements and declarations: An expressionstatement with a function-style explicit type conversion (5.2.3) 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. This disambiguates many examples. [ Example: assuming T is a simple-type-specifier (7.1.6),

T(a)->m = 7; // expression-statement
T(a)++; // expression-statement
T(a,5) << c; // expression-statement

T(*d)(int); // declaration
T(e)[5]; // declaration
T(f) = { 1, 2 }; //declaration
T(*g)(double(3)); // declaration

In the last example above, g, which is a pointer to T, is initialized to double(3). This is of course ill-formed for semantic reasons, but that does not affect the syntactic analysis. —end example ]

How to fix it

All we need is something that makes it impossible for the compiler to treat the code as a function declaration.

1. Adding the extra parentheses around the parameters makes it clear to the compiler that what we intend to be constructor parameter names can't be parameter declarations.

   vector<int> vec((istream_iterator<int>(cin)), istream_iterator<int>());

2. As your solution showed, using named variables as constructor parameters.