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  1. #include <utility>
  2.  
  3. #define EXEC_MEMBER_PROC_IF_PRESENT(ProcName) namespace ProcName {\
  4.   void ExecIfPresent(...) throw() {}\
  5.   \
  6.   template <class C, typename... TArgs>\
  7.   void ExecIfPresent(C& obj, TArgs&... args) {\
  8.     obj.ProcName(std::forward<TArgs>(args)...);\
  9.   }\
  10. };
  11.  
  12. // If NOT exists - returns the 'DefaultValue'
  13. // 'DefaultValue' SHOULD be the same type as a decltype(*.FuncName())
  14. // Works with static/const/virtual funcs
  15. #define EXEC_MEMBER_FUNC_IF_PRESENT(FuncName, DefaultValue) namespace FuncName {\
  16.   template <typename TReturnType = decltype(DefaultValue)>\
  17.   auto ExecIfPresent(...) -> TReturnType {\
  18.     return std::move(DefaultValue);\
  19.   }\
  20.   \
  21.   template <class C, typename... TArgs>\
  22.   auto ExecIfPresent(C& obj, TArgs&... args)\
  23.    -> decltype(obj.FuncName(std::forward<TArgs>(args)...))\
  24.   {/* do NOT use 'const C& obj' NOR 'C::FuncName()!'*/\
  25.     return std::move(obj.FuncName(std::forward<TArgs>(args)...));\
  26.   }\
  27. };
  28.  
  29. EXEC_MEMBER_FUNC_IF_PRESENT(getHashIfKnown, 0U);
  30.  
  31. #include<cassert>
  32. #include<cstring>
  33. #include<string>
  34. #include<iostream>
  35.  
  36. int main() {
  37.  std::string str = "test str";
  38.  auto hash = getHashIfKnown::ExecIfPresent(str);
  39.  assert(!hash);
  40.  std::cout << "str: " << hash << std::endl;
  41.  
  42.  struct MyStr {
  43.  	MyStr(const char* str) throw() {
  44.  		if(str) {
  45.  			strcpy(buf, str);
  46.  			len = strlen(buf);
  47.  		}
  48.  	}
  49.  
  50.  	size_t getHashIfKnown() const throw() {
  51.  		size_t hash = 0U;
  52.  		for(size_t idx = 0U; idx < len; ++idx) {
  53.  			hash += buf[idx] * (idx + 1U);
  54.  		}
  55.  		return hash;
  56.  	}
  57.  
  58.  	size_t len = 0U;
  59.  	char buf[256U];
  60.  } myStr = "we don't need no water!";
  61.  
  62.  hash = getHashIfKnown::ExecIfPresent(myStr);
  63.  assert(hash);
  64.  std::cout << "myStr: " << hash << std::endl;
  65. }
Success #stdin #stdout 0s 3456KB
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stdin
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Standard input is empty
stdout
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str: 0
myStr: 24355
https://ideone.com/Afh18i
language:
C++14 (gcc 8.3)
created:
8 years ago
visibility:
public

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