I have a class called variableMap which keeps track of variable values in a calculator:

//parser.h
namespace parser
{
    class VariableMap
    {
        private:
            std::unordered_map<std::string, std::string> variables{};
            std::vector<Token> replaceVars(std::vector<Token> tokens) const;

        public:
            double parseExp(std::vector<Token> tokens);      
    };
}

From posts like this I have learned that it is good practice to keep functions that does not handle data members of a class as separate functions within a namespace. So I implemented a couple of functions that does not directly deal with the variables map inside the namespace parser:

//parser.cpp
namespace parser
{
    void displayTokens(const std::vector<Token>& tokens)
    {
        // does stuff
    }
    
    namespace
    {
        std::string sumTokens(const std::vector<Token>& tokens) 
        {
            // does stuff
        }
    }
}

The problem is that functions like sumTokens() are dependent on the non-static method replaceVars(). Meaning I would somehow have to give sumTokens() access to the objects of VariableMap. Is it not just better to make sumTokens() a static method of the class in scenarios like these?