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  1. #include <iostream>
  2. #include <vector>
  3. #include <array>
  4. using namespace std;
  5.  
  6. int main()
  7. {
  8. 	std::vector<int>    vec100(100);  // a vector of 100 ints
  9. 	std::vector<int>    vec2(2);      // a vector of 2 ints
  10. 	std::array<int,100> arr100;       // an array of 100 ints
  11. 	std::array<int,2>   arr2;         // an array of 2 ints
  12.  
  13. 	// notice how the number of elements does not change the size
  14. 	//  of the vector itself:
  15. 	cout << "sizeof vec100 = " << sizeof(vec100) << endl;
  16. 	cout << "sizeof vec2   = " << sizeof(vec2) << endl;
  17.  
  18. 	// This is because the vector does not contain the elements, but rather
  19. 	//   it contains a pointer to them.  This means to access the elements,
  20. 	//   it has to dereference that pointer, which means an extra memory access
  21.  
  22.  
  23. 	// On the other hand... notice how the number of elements does
  24. 	//  change the size of an array:
  25. 	cout << "sizeof arr100 = " << sizeof(arr100) << endl;
  26. 	cout << "sizeof arr2   = " << sizeof(arr2) << endl;
  27.  
  28. 	// This is because the array does actually contain its elements and not
  29. 	//  merely a pointer to them.  This allows it to avoid that extra memory
  30. 	//  access and thus perform slightly better in high-demand situations.
  31. 	//
  32. 	// Note that this is only possible in array because it knows the size
  33. 	//  at COMPILE TIME and therefore it cannot resize it at runtime.
  34. 	//
  35. 	// Vector cannot do this because it needs to be able to adjust the size
  36. 	//  at runtime.  Any dynamically sized container is going to have the
  37. 	//  same issue.
  38. 	return 0;
  39. }
Success #stdin #stdout 0s 3428KB
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stdin
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Standard input is empty
stdout
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sizeof vec100 = 12
sizeof vec2   = 12
sizeof arr100 = 400
sizeof arr2   = 8
https://ideone.com/PY8BOm
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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