std::vector size?

Question

Program: ``` #include int main() { std::vector::size_type size=3; std::vector 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::size_type {aka long unsigned int}’ to ‘int’ inside { } [-fpermissive] ppp.cpp:5:28: error: narrowing conversion of ‘size’ from ‘std::vector::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?

Accepted Answer

You're not calling the constructor that sets the vector to an initial size. ``` std::vector 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 init, const Allocator& alloc = Allocator() ); ``` The braced-initializer list gets deduced as an `std::initializer_list` and then a narrowing conversion must be performed since the `vector` itself contains `int`s. To set the initial size of the vector use: ``` std::vector 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.

std::vector size?

Solution

Problem