laoqiren - 1 year ago 79
C++ Question

# vector in C++ can only read the last element

I add several element to the vector, but when I visit them, they are all the one which is added at last. I can't understand.

here is my code:

``````while(true){
cin>>shape;
if(shape=='X') break;
if(shape=='C'){
cin>>x>>y>>r;
Circle c(x,y,r);
shapes[sum] = &c;
//cout<<shapes[sum]->getArea()<<endl;
sum++;
}else if(shape=='R'){
cin>>x1>>y1>>x2>>y2;
Rectangle rec(x1,y1,x2,y2);
shapes[sum] = &rec;
//cout<<shapes[sum]->getArea()<<endl;
sum++;
} else if(shape=='T'){
cin>>x1>>y1>>x2>>y2>>x3>>y3;
Triangle tr(x1,y1,x2,y2,x3,y3);
shapes[sum] = &tr;
//cout<<shapes[sum]->getArea()<<endl;
sum++;
}
}
for(int j=0; j<sum; j++){
showArea(shapes[j]);
}
``````

I found that at last, all the element in the vector is same, they are the element which is added at last.

Your vector stores pointer. And you store inside it pointer of local variables:

``````} else if(shape=='T'){
cin>>x1>>y1>>x2>>y2>>x3>>y3;
Triangle tr(x1,y1,x2,y2,x3,y3); // <= Create local variable, automatic allocation
shapes[sum] = &tr; // <= store its address
//cout<<shapes[sum]->getArea()<<endl;
sum++;
} // <= automatic dealocation of tr, ie tr doesn't exist anymore
// shapes[sum - 1] stores address of no more existing variable => Undefined behavior
``````

you should do:

``````} else if(shape=='T'){
cin>>x1>>y1>>x2>>y2>>x3>>y3;
Triangle *tr = new Triangle(x1,y1,x2,y2,x3,y3); // manual allocation
shapes.push_back(tr);
//cout<<shapes[sum]->getArea()<<endl;
sum++;
}
``````

BUT you have to deallocate with `delete` when you don't need anymore objects in vector

`sum` isn't necessary: you have to use `push_back` to avoid Undefined Behavior, and after, you can use `shapes.size()` to retrieve size of vector.

Indeed, accessing to an element of vector which is out of bound (ie when you do `vector[n]`, `n` is equal or greater than `vector.size()`) is undefined behavior.

Modern way of doing this: use of `shared_ptr` or `unique_ptr`

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download