Thread: Vector<char> push_back

1. How to append at the end of one vector<char> another one with push_back?
I know that v.insert() works too but I wonder if it can be done with push_back.
2. Does it matter if I declare vector<unsigned char> or just vector<char> if I want to store binary string in it?

Code:

int main()
{
     vector<char> v(5, '0');
     vector<char> v2;

     v2.push_back(&v.at(0));
     v2.push_back(v);
}

Originally Posted by Ducky

1. How to append at the end of one vector<char> another one with push_back?
I know that v.insert() works too but I wonder if it can be done with push_back.

push_back() appends only a single element so use a loop calling push_back() for each element of the other vector you intend to append. Probably easier to use insert() though.

Originally Posted by Ducky

2. Does it matter if I declare vector<unsigned char> or just vector<char> if I want to store binary string in it?

It depends on how you have obtained that "binary string", and what you intend to do with it after storing it. If you read it from a standard ifstream (assuming the stream has been opened in binary mode), data is typically read as char.

It is implementation defined whether a (unadorned) char is signed or unsigned. If it is signed, the behaviour is undefined on overflow and underflow (e.g. what happens if a char holds its maximum possible value and it is then incremented). If char is unsigned, the behaviour is well defined on overflow (i.e. modulo arithmetic). If you will do operations on the data that potentially overflow the individual characters, it is often considered better to work with unsigned char.

Thanks Grumpy.
1. I want to append only one part of a vector<char> at the end of the other so I better use a loop to get the part I want to copy I guess cause I dont think that insert() can do this.

2. Since Vector is not a pointer I guess there is no risk of overflow.

I think you can use resize() and std::copy

Originally Posted by Ducky

1. I want to append only one part of a vector<char> at the end of the other so I better use a loop to get the part I want to copy I guess cause I dont think that insert() can do this.

Yes it can. One of the forms of insert() accepts two input iterators that specify the beginning and end of an input range.

Originally Posted by Ducky

2. Since Vector is not a pointer I guess there is no risk of overflow.

You clearly misunderstood the point made. I did not refer in any way to a vector or pointer overflowing. I referred to the possibility of the value of characters overflowing.

Code:

#include <limits>

int main()
{
     char c = std::numeric_limits<char>::max();
     ++c;    //  This overflows c.  Results are undefined if char is signed

     unsigned char u = std::numeric_limits<unsigned char>::max();
     ++u;    //  this overflows u.   The result is zero since u is of type unsigned char
}

As I said, what works depends on what you want to do with the characters in the vector.