CppCompileError_algoCcannotModifyAconstObject.md

 

 

 

 

 

(C++) _algo.c: Cannot modify a const object

 

Compile error.

 

 

 

 

 

Full error message

 

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[C++ Error] _algo.c(151): E2024 Cannot modify a const object

 

The compiler takes you to the following line in _algo.c:

 

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// search_n. Search for __count consecutive copies of __val.   template <class _ForwardIter, class _Integer, class _Tp> _ForwardIter search_n(_ForwardIter __first, _ForwardIter __last,                       _Integer __count, const _Tp& __val) {   _STLP_DEBUG_CHECK(__check_range(__first, __last))   if (__count <= 0)     return __first;   else {     __first = find(__first, __last, __val);     while (__first != __last) {       _Integer __n = __count - 1;       _ForwardIter __i = __first;       ++__i;       while (__i != __last && __n != 0 && *__i == __val) {         ++__i;         --__n; // <---THIS LINE       }       if (__n == 0)         return __first;       else         __first = find(__i, __last, __val);     }     return __last;   } }

 

 

 

 

 

Cause

 

IDE: C++ Builder 6.0

Compiler: Borland BCC32.EXE version 6.0.10.157

Project type: Console Application

 

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#include <algorithm> #include <string> int main() {   const std::string s = "abc***def";   const int n = 3; //Number of repeats   std::search_n( s.begin(),s.end(),n,'*'); }

 

 

 

 

 

Solution/workaround

 

Remove the const of the int for the number of repeats, by static_casting it in the function call.

 

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#include <string> #include <algorithm> int main() {   const std::string s = "abc***def";   const int n = 3; //Number of repeats   std::search_n( s.begin(),s.end(),static_cast<int>(n),'*'); //n must be copied to int type }

 

Note that a const_cast does not work. Personally, I would find this more appropriate, but I do not understand why this keeps giving the same error.

 

 

 

 

 

In-depth cause and better solution for advanced programmers

 

The actual problem is in _algo.c. I have made all relevant information strong:

 

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// search_n. Search for __count consecutive copies of __val.   template <class _ForwardIter, class _Integer, class _Tp> _ForwardIter search_n(_ForwardIter __first, _ForwardIter __last,                       _Integer __count, const _Tp& __val) {   _STLP_DEBUG_CHECK(__check_range(__first, __last))   if (__count <= 0)     return __first;   else {     __first = find(__first, __last, __val);     while (__first != __last) {       _Integer __n = __count - 1;       _ForwardIter __i = __first;       ++__i;       while (__i != __last && __n != 0 && *__i == __val) {         ++__i;         --__n;       }       if (__n == 0)         return __first;       else         __first = find(__i, __last, __val);     }     return __last;   } }

 

Because _Integer is a template type, the constness of the __count argument is also taken into account. The local _Integer __n, however, must not be const.

 

A better solution would be to make __n of non-const __integer type, so the user can write const-correct code.