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talk:CppVector

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1th March 2007, Bilderbikkel

The code under paragraph 'Memory use' was changed by Jvorwald to this:

include <iostream>

include <vector>


int main(int argc, char *argv[])
{ 
  const unsigned int smallSize = 10; 
  const unsigned int bigSize = 10000; 
  const size_t nbytes_in_int(sizeof(int));
  const size_t nbytes_in_char(sizeof(char));
  int smallArray[smallSize]; 
  int bigArray[bigSize]; 
  std::vector<int> smallVector(smallSize); 
  std::vector<int> bigVector(bigSize); 
  
  std::cout << "char: " << nbytes_in_char << std::endl; 
  std::cout << "int: " << nbytes_in_int << std::endl; 
  std::cout << "Small array: " << sizeof(smallArray) << std::endl; 
  std::cout << "Big array: " << sizeof(bigArray) << std::endl; 
  std::cout << "Small vector: " << sizeof(smallVector) 
            << " + " << smallSize << " * " << sizeof(smallVector[0]) << std::endl; 
  std::cout << "Big vector: " << sizeof(bigVector) 
            << " + " << bigSize << " * " << sizeof(bigVector[0]) << std::endl; 
            
  for (size_t ii(0); ii<bigSize; ii++) bigVector[ii] = ii*2;
  
  const size_t nbytes(sizeof(bigVector) + bigSize * sizeof(bigVector[0]));
  const size_t nchar(nbytes/nbytes_in_char);
  
  char cc[nchar];
  char *cptr;
  cptr = (char *)(&bigVector);
  for (size_t ii(0); ii<nchar; ii++) cc[ii] = cptr[ii];
  
  std::vector<int> *vptr;
  vptr = (std::vector<int> *) cc;
  
  if ((*vptr)==bigVector)
  {
    std::cout << "  Copied into char array correctly" << std::endl;
    std::cout << "    &bigVector = " << &bigVector << ", vptr = " << vptr << std::endl;
    for (size_t ii(0); ii<bigVector.size(); ii+=1000) 
      std::cout << "    bigVector[" << ii << "] = " << bigVector[ii]
                << "    (*vptr)[" << ii << "] = " <<  (*vptr)[ii]
                << std::endl;
  }
  else
  {
    std::cout << "  Did not copy into char array correctly" << std::endl;
  }
  
  std::cout << std::endl << "Press anything to exit";
  char ch;
  std::cin.clear();
  std::cin.get(ch);
  return EXIT_SUCCESS;
} 

That the paragraph was modified, however, made clear that it was not understood which point was made. I, Bilderbikkel, added some text describing what I wanted to show:

• a static array of size 100 is 10x bigger then a static array of size 10
• a std::vector of size 100 is as big as a std::vector of size 10

Of the code added, it can be questioned what the point of the author was to make. If the copying of a std::vector to an array and back was to be demonstrated, this could be done better in a seperate paragraph or at the page CppCopy, the page that described the std::copy command.

The coding style can also be debated:

• The code generates an access violation
• Why use 'int main(int argc, char *argv[])' if 'int main()' is also standard C++ when you do not use argc and argv to make a point?
• Why 'return EXIT_SUCCESS' when the Standard states that the closing bracket of main() must have the same effect as 'return 0;' ?
• The code uses non-standard variable names: why use 'ii' when 'i' is 1000x more common?
• Why use 'i++' when '++i' is to be preferred?
• Why use 'char ch; std::cin.clear(); std::cin.get(ch);' when a std::cin.get() does the job as well?

For references on my critique, see e.g. CppMain and CppFor.
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