c++ - How do I fix the linker error? -

in function main':
[linker error] undefined reference tobinary_to_decimal()'
[linker error] undefined reference `decimal_to_binary()'
working on decimal binary , binary decimal program. unfortunately ran compiling errors lack knowledge fix. appreciate fix , learn issue.
here source code of program.

#include <iostream>  #include <string>  #include <bitset>  void binary_to_decimal();  void decimal_to_binary();    int main (int argv, char argc) {        while(1<2){       int m_choice;        std::cout << "enter 1 - binary decimal" << std::endl;        std::cout << "enter 2 - decimal binary" << std::endl;        std::cin >> m_choice;       if (m_choice == 1) {                     binary_to_decimal();                     }else if (m_choice == 2) {                          decimal_to_binary();                            }          return 0;       }  }        void binary_to_decimal(){       std::string binary_to_decimal_cstr;      std::cout << "please enter binary number: " << std::endl;       std::cin>>binary_to_decimal_cstr;       std::cout<<binary_to_decimal_cstr;       std::cout <<"converted decimal is:" << std::bitset<32>(binary_to_decimal_cstr).to_ulong();       std::cout << std::endl;       }   void  decimal_to_binary(){            int decimal_to_binary_var;          std::cout << "please enter  decimal number: " << std::endl;          std::cin >> decimal_to_binary_var;          std::cout << decimal_to_binary_var;         std::cout << "converted binary is: " << std::bitset<32>(decimal_to_binary_var);          std::cout << std::endl;           } 

case sensitive problem!

void binary_to_decimal();  --> void binary_to_decimal();              ^                              ^   void decimal_to_binary();  --> void decimal_to_binary();               ^                              ^ 

compiler searches void binary_to_decimal() implelemted void binary_to_decimal(); different thing.