Overview

Optimizing in C++ means knowing your environment. One should know how vector works, and vector works with pushing back elements into an array. 

Advantages of std::vector

Dynamic arrays in C++ are implemented using std::vector.

When a new element is inserted, and the existing capacity of the vector is exhausted, the std::vector resizes itself. The way it does that is by allocating memory at a different location that is large enough to fit the element we are inserting as well as the existing items. Then it copies all the existing items into the new location and adds the element we are trying to insert.

The standard vector resizes itself very frequently when elements get pushed into it. It results in multiple memory reallocations and slower, inefficient code. To avoid resizing and copying, we can use the reserve function. If we know how many elements the vector will hold, we can reserve that size at the beginning. Reserve will make sure that we have enough memory. Thus, multiple memory reallocations will be avoided whenever new elements are inserted into the vector.

How is this feature integrated with Embold?

Embold has integrated a checker in C++. 

• performance-inefficient-vector-operation: It finds possible inefficient std::vector operations (e.g.: push_back) that may cause unnecessary memory reallocations.

Example: 

std::vector v;
for (int i = 0; i < n; ++i) {
v.push_back(n);
}

An explanation for the above example

The push_back function may cause multiple memory reallocations in v. Avoid by inserting a ‘reserve(n)‘ statement before the ‘for’ statement.

std::vector<int> v;
v.reserve(n);
for (int i = 0; i < n; ++i) {
v.push_back(n);
}

Conclusion

We are spending too much time on reallocating and deallocating memory. However, by using the C++ function std::vector::reserve, it helps to reserve the vector capacity that may contain at least n elements.

Also, it is recommended the usage of std::vector in most cases where dynamic arrays are needed.  And this optimizing by using a standard vector will lead to easily resizing the length.