CODEWITHSUNDEEP
c++vector
eXPRESS
eXPRESS

Reputation: 465

Vector is not "saving values" C++

When reading from vector I put values in I got zero size. I have:

class Graph {
public:
    vector<Vertex> vertices;
};

class Vertex {
public:
    vector<int> adjacentVertices;
};

In my load method then:

int vertices, edges;
cin >> vertices >> edges;
Graph mainGraph;
mainGraph.vertices.reserve(static_cast<unsigned int>(vertices));
int tmp1, tmp2;
for (int i = 0; i < edges; i++) {
    cin >> tmp1 >> tmp2;
    mainGraph.vertices[tmp1].adjacentVertices.push_back(tmp2);
    cout << mainGraph.vertices[tmp1].adjacentVertices.size(); //PRINTS NUMBERS -> SEEMS OKAY
}

cout << mainGraph.vertices.size(); //IS ZERO???

for(const Vertex &v : mainGraph.vertices){ //CRASHES
    cout << v.adjacentVertices.size();
}

I bet this is very stupid but what am I missing? I read the vector will self construct itself upon use if no special constructor is needed.

Upvotes: 2

Views: 1435

Answers (3)

Sceptical Jule
Sceptical Jule

Reputation: 917

That happened because you're trying to manage the size of your vectors manually. Let the STL do all the work dynamically:

Graph mainGraph;
int edge;

while (cin >> edge)
{
    Vertex newVertex;
    newVertex.adjacentVertices.push_back(edge);
    mainGraph.vertices.push_back(std::move(newVertex));
}

for(const Vertex &v : mainGraph.vertices)
    cout << v.adjacentVertices.size();

https://ideone.com/BQq01W

Upvotes: 0

gsamaras
gsamaras

Reputation: 73366

Change this:

mainGraph.vertices.reserve(static_cast<unsigned int>(vertices));

to this:

mainGraph.vertices.resize(static_cast<unsigned int>(vertices));

since with reserve() the cells of the vector are not created, which can be done though with resize().

As a result, in your code, you invoke Undefined Behavior (UB), when you do mainGraph.vertices[tmp1], since you attempt to access an object that has not been constructed!

Upvotes: 8

Rotem
Rotem

Reputation: 21927

reserve does not change the size of the vector or construct items. It only allocates memory that will be reserved for when you do add elements.

Your call to mainGraph.vertices[tmp1] is undefined as it is accessing raw memory.

Instead of reserve, use resize to both allocate and construct items.

Upvotes: 7

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