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  1. /* This doc is to investigate the issue brought by reference return type of a
  2.  * functor argument std::function. We expect it can pass the bypass function
  3.  * defined in top layer to less-context bottom layer and still work as designed.
  4.  * However we see weird behavior when it is std::function<const std::string&(int)>.
  5.  * Instead, std::function<const string*(int)> works fine...
  6. */
  7. #include <iostream>
  8. #include <vector>
  9. #include <unordered_map>
  10. #include <string>
  11. #include <functional>
  12.  
  13. using namespace std;
  14.  
  15. // This class stores vectror of numbers and divisors. API getRemainderRing picks
  16. // those numbers' remainder equal to given number after division. Bypass function
  17. // is passed in as argument to print the names.
  18. class Elements {
  19. public:
  20.     Elements() = default;
  21.     Elements(int32_t maxNum, int32_t divisor) : _div(divisor) {
  22.         _data.clear();
  23.         _data.reserve(maxNum);
  24.         for (int32_t i = 0; i < maxNum; i++) {
  25.             _data.push_back(i);
  26.         }
  27.     }
  28.  
  29.     void getRemainderRing(int32_t rmd, const std::function<const string&(int32_t)>& getName, string* output) {
  30.         output->clear();
  31.         for (int32_t i : _data) {
  32.             if (i % _div == rmd) {
  33.                 // crashes here. getName(i) pointing to address 0
  34.                 *output += getName(i) + " ";
  35.             }
  36.         }
  37.     }
  38.  
  39. private:
  40.     vector<int32_t> _data;
  41.     int32_t _div;
  42. };
  43.  
  44. int main () {
  45.     unordered_map<int32_t, string> numToStr;
  46.     numToStr[0] = "null";
  47.     numToStr[1] = "eins";
  48.     numToStr[2] = "zwei";
  49.     numToStr[3] = "drei";
  50.     numToStr[4] = "vier";
  51.  
  52.     // The functor
  53.     std::function<const string&(int32_t)> getName = [&numToStr](int32_t i) -> const string& { return numToStr[i]; };
  54.  
  55.     Elements smallRing(4, 2); // contains {0,1,2,3}, divisor: 2
  56.     string result;
  57.     // This is actually to get all odd numbers < 4
  58.     smallRing.getRemainderRing(1, getName, &result);
  59.  
  60.     // BOOM!
  61.     cout << result << endl;
  62.  
  63.     return 0;
  64. }
Success #stdin #stdout 0s 4536KB
comments (?)
stdin 
Standard input is empty
stdout 
eins drei
https://ideone.com/Z0T1MK
language:
C++ (gcc 8.3)
created:
6 years ago
visibility:
public

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