std::vector size?

Question

Program:

#include<vector>

int main() {
    std::vector<int>::size_type size=3;
    std::vector<int> v{size};
}

when compiled with

g++ (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3

generates error:

ppp.cpp: In function ‘int main()’:
ppp.cpp:5:28: error: narrowing conversion of ‘size’ from ‘std::vector<int>::size_type {aka long unsigned int}’ to ‘int’ inside { } [-fpermissive]
ppp.cpp:5:28: error: narrowing conversion of ‘size’ from ‘std::vector<int>::size_type {aka long unsigned int}’ to ‘int’ inside { } [-fpermissive]

and on http://www.cplusplus.com/reference/stl/vector/vector/ it is written

explicit vector ( size_type n, const T& value= T(), const Allocator& = Allocator() );

I expected that constructor to be used.

Can somebody explain?

Answer 1

You're not calling the constructor that sets the vector to an initial size.

std::vector<int> v{size};

The above creates a vector containing a single int element with the value size. You're calling this constructor:

vector( std::initializer_list<T> init, const Allocator& alloc = Allocator() );

The braced-initializer list gets deduced as an std::initializer_list<size_type> and then a narrowing conversion must be performed since the vector itself contains ints.

To set the initial size of the vector use:

std::vector<int> v(size);  // parentheses, not braces

---

Also, the vector constructor you've listed no longer exists, it was removed in C++11 and replaced by the following two constructors:

vector( size_type count, const T& value, const Allocator& alloc = Allocator());

explicit vector( size_type count );

cppreference.com is a much better reference as compared to cplusplus.com.