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V827. Maximum size of a vector is known…
Analyzer diagnostics
General Analysis (C++)
General Analysis (C#)
General Analysis (Java)
Diagnosis of micro-optimizations (C++)
Diagnosis of 64-bit errors (Viva64, C++)
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Contents

V827. Maximum size of a vector is known at compile time. Consider pre-allocating it by calling reserve(N).

23 Jul 2020

The analyzer has detected an 'std::vector' whose maximum size is known at compile time and the 'reserve' method is not called before filling it.

Consider the following example:

void f()
{
  std::vector<int> v;
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);
  v.push_back(4);
  v.push_back(5);
  v.push_back(6);
}

In this case, the calls to 'push_back' may lead to reallocating the vector's internal buffer and moving the elements to a new memory block.

To reduce the overhead, we could have a buffer of an appropriate size pre-allocated:

void testVectOK()
{
  std::vector<int> v;
  v.reserve(6);

  v.push_back(1);
  v.push_back(2);
  v.push_back(3);
  v.push_back(4);
  v.push_back(5);
  v.push_back(6);
}

The analyzer's warning includes the number of elements to pass to the 'reserve' method.

It is sometimes impossible for the analyzer to calculate the exact size of the container. This happens, for example, when elements are added based on a condition:

void f(bool half)
{
  std::vector<int> v;
  v.push_back(1);
  v.push_back(2);
  v.push_back(3);
  
  if (!half)
  {
    v.push_back(4);
    v.push_back(5);
    v.push_back(6);
  }
}

Here, the number of elements in the container can be either 3 or 6 depending on the condition. In cases like that, the analyzer will suggest the maximum size possible.

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