c++ vector

bendeco13 · 11-04-2005, 12:23 AM

This may be a bit of a dumb question, considering I don't think there is any way to do what I'm wanting, but I guess thats what questions are for.

I have a program and I want to define a list of strings. I can do this as an array, but then I can't determine the size of the array at a later time. So I decided I would have to use vectors.

I tried this, but it surely didn't work.

Code:

vector<string> path_column_names = {"Name", "Last Changed"};
vector<string> file_column_names = {"Name", "Size", "Last Changed"};

I know I could define the two and then use push_back(), but I don't want to do this, because I want others to be able to easily (see) and modify this at a later time.
Is there some other way of doing this without using the push_back() method?

Thanks in advance,
Bendeco

YetAnotherDave · 11-04-2005, 09:34 AM

How about something like this? It's not exactly what you want but perhaps better than push_back() .

Code:

#define SPAN(Z) (Z), ((Z)+sizeof(Z)/sizeof(Z[0]))

char* pc[] = {"Name", "Last Changed"};
vector<string> path_column_names(SPAN(pc));

char* fc[] = {"Name", "Size", "Last Changed"};
vector<string> file_column_names(SPAN(fc));

- Dave

bendeco13 · 11-04-2005, 12:42 PM

That does exactly what I wanted.
Thanks alot ...

If you don't mind me asking, I'm a little new to C++, can you describe what the
#define SPAN(Z) (Z), ((Z)+sizeof(Z)/sizeof(Z[0]))
actually does/means?

I know what the preprocessor directive define does, I'm just wandering about everything else.
Thanks

YetAnotherDave · 11-04-2005, 01:32 PM

The expression "sizeof(Z)/sizeof(Z[0])" in SPAN(Z) is the number of elements in the Z array.

The "(Z)" part of SPAN(Z) is a pointer to the first entry in the Z array and the "((Z)+sizeof(Z)/sizeof(Z[0]))" part points one element past the last entry in the array.

So far this looks complicated but I think it will work even in standard C ( I didn't try it though ).

A pointer will work as an stl iterator so constructing a vector with the two pointers result in the following vector constructor being called:

Code:

template <class InputIterator> 
vector(InputIterator, InputIterator)

which results in a vector being created from the objects that the iterators/pointers from the first argument ( inclusive ) to the last argument ( non-inclusive ) point to.

Here's the same thing without the use of the macro and with an intermediate variable which might make things clearer:

Code:

char* pc[] = {"Name", "Last Changed"};
unsigned int pcLen = sizeof(pc)/sizeof(pc[0]);  // this evaluates to 2
vector<string> path_column_names(pc, pc+pcLen);

I didn't compile the code in this response so there may be typos.

- Dave

bendeco13 · 11-04-2005, 02:22 PM

Thanks again...
That pretty much clears it all up.
It's people like you who keep us inspired to program.