no instance of overloaded function std::unordermap.insert

Question

I declared a stream class, but when defining its member function, it always reports an error when I want to insert something into the private member unordered_map, I want to ask How to solve this? stream.h

#include <unordered_map> class stream { private: std::unordered_map<size_t, std::string> word_count; public: void reassemblestream() const; };

stream.cpp

#include"stream.h" using namespace std; void stream::reassemblestream() const { static size_t a = 1; string fe = "dfi"; word_count.insert({ 1,"dfs" }); word_count.insert(make_pair(a, fe)); }

error happen in stream.cpp of word_count.insert funcion. This is the error information:

E1087 There is no overloaded function "std::unordered_map<_Kty, _Ty, _Hasher, _Keyeq, _Alloc>::insert [where _Kty=size_t, _Ty=std::string, _Hasher=std:: hash<size_t>, _Keyeq=std::equal_to<size_t>, _Alloc=std::allocator<std::pair<const size_t, std::string>>]" instance (object contains type qualifiers that prevent matching)

I use visual studio 2019.

You marked the member function const, but then you try to modify the object. — molbdnilo
– molbdnilo, Commented Aug 10, 2021 at 8:30
You can make word_count mutable, but that defeats the purpose — Lala5th
– Lala5th, Commented Aug 10, 2021 at 8:32
You're not stupid, it's a vague and misleading error message. (It looks like an IntelliSense message, and those are often not very good. You might get a better message if you compile the code.) — molbdnilo
– molbdnilo, Commented Aug 10, 2021 at 8:37

prehistoricpenguin · Accepted Answer · 2021-08-10 08:43:35Z

If build the program with clang++, we get more clear error messages:

source>:17:16: error: no matching member function for call to 'insert' word_count.insert({ 1,"dfs" }); ~~~~~~~~~~~^~~~~~ /opt/compiler-explorer/gcc-11.1.0/lib/gcc/x86_64-linux-gnu/11.1.0/../../../../include/c++/11.1.0/bits/unordered_map.h:557:7: note: candidate function not viable: 'this' argument has type 'const std::unordered_map<size_t, std::string>' (aka 'const unordered_map<unsigned long, basic_string<char>>'), but method is not marked const insert(value_type&& __x)

Just remove the const qualifiers, since you are modifying the data member by calling the insert function, the function reassemblestream can't be marked as const(related question for const member function):

#include <unordered_map> #include <string> class stream { private: std::unordered_map<size_t, std::string> word_count; public: void reassemblestream(); }; void stream::reassemblestream() { static size_t a = 1; std::string fe = "dfi"; word_count.insert({ 1,"dfs" }); word_count.insert(make_pair(a, fe)); }

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