Program killed: using vector of set of vector - c++

Question

In my Algorithm, I need to keep all the combinations of (3 bytes of) extended ASCII characters. Following is my code But when i run this code, the program gets killed on terminal when the last step occurs(BigVector.pushback).Why is this so and what can be the alternative in my case?
vector<set<vector<int> > > BigVector;
set<vector<int> > SmallSet;
for(int k=0; k <256; k++)
{
for(int j=0; j <256; j++)
{
for(int m=0; m <256; m++)
{
vector<int> temp;
temp.push_back(k);
temp.push_back(j);
temp.push_back(m);
SmallSet.insert(temp);
}
}
}
BigVector.push_back(SmallSet);
P.S: I have to keep the ascii characters like this:
{ {(a,b,c) ,(a,b,d),...... (z,z,z)} }

Answer 1

Please note that 256^3 = 16,777,216. This is huge, especially when you use vector and set!
Because you only need to record 256 = 2^8 information, you can store this in a char ( one byte). You can store each combination in one tuple of three chars. The memory is now 16,777,216 / 1024 / 1024 = 16 MB. On my computer, it finishes in 1 second.
If you accept C++11, I would suggest using std::array, instead of writing a helper struct like Info in my old code.
C++11 code using std::array.
vector<array<char,3>> bs;
.... for loop
array<char,3> temp;
temp[0]=k; temp[1]=j; temp[2]=m;
bs.push_back(temp);
C++98 code using home-made struct.
struct Info{
char chrs[3];
Info ( char c1, char c2, char c3):chrs({c1,c2,c3}){}
};
int main() {
vector<Info> bs;
for (int k = 0; k < 256; k++) {
for (int j = 0; j < 256; j++) {
for (int m = 0; m < 256; m++) {
bs.push_back(Info(k,j,m));
}
}
}
return 0;
}
Ways to use the combinations. (You can write wrapper method for Info).
// Suppose s[256] contains the 256 extended chars.
for( auto b : bs){
cout<< s[b.chrs[0]] << " " << s[b.chrs[1]] << " "<< s[b.chrs[2]] << endl;
}

Answer 2

First: your example doesn't correspond with the actual code.
You are creating ( { (a,a,a), ..., (z,z,z) } )
As already mentioned you will have 16'777'216 different vectors. Every vector will hold the 3 characters and typically ~20 bytes[1] overhead because of the vector object.
In addition a typical vector implementation will reserve memory for future push_backs.
You can avoid this by specifying the correct size during initialization or using reserve():
vector<int> temp(3);
(capacity() tells you the "real" size of the vector)
push_back makes a copy of the object you are pushing [2], which might be too much memory and therefore crashing your program.
16'777'216 * (3 characters + 20 overhead) * 2 copy = ~736MiB.
(This assumes that the vectors are already initialized with the correct size!)
See [2] for a possible solution to the copying problem.
I do agree with Potatoswatter: your data structure is very inefficient.
[1] What is the overhead cost of an empty vector?
[2] Is std::vector copying the objects with a push_back?