Re: Implement ORDER BY clause in c++

Tamas Demjen <>
Fri, 26 May 2006 18:14:22 -0700
Index wrote:

I have a file which I want to sort depending on multiple columns.

It depends where your data are located, and how they are represented. If
your records fit in the memory, you could store them in an STL vector,
then define a custom ordering operator.

Here's how to create a function object that orders by two fields (first
by "priority", and if they are the same, then by "size"):

struct Record
    int priority;
    int size;
    [...] // other members

struct OrderByPriorityThenSize
    bool operator()(const Record& lhs, const Record& rhs) const
       if(lhs.priority == rhs.priority)
          return lhs.size < rhs.size; // secondary ordering
          return lhs.priority < rhs.priority; // primary ordering

Then assuming you have a vector of Record's, you can sort them using
your defined order:

std::vector<Recrod> items;
[...] // load the items
std::sort(items.begin(), items.end(), OrderByPriorityThenSize());
[...] // save the items

There are some ultra lightweight database engines available, such as
SQLite. That way you can use SQL queries to manage your data.

I'm not sure if this answers your question. If not, you'll need to
provide more details about your project.


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