c++ 多个重载函数实例匹配参数类型

我已经重载了我的函数contains三遍

// returns true if char c is contained in unordered map um
bool contains(std::unordered_map<char, op>& um, char c){
    return um.find(c) != um.end();
}

// returns true if string s is contained in unordered map um
bool contains(std::unordered_map<char, op>& um, std::string& s){
    return s.length() == 1 && contains(um, s[0]); 
}

// returns true if string s is contained in unordered map um
bool contains(std::unordered_map<std::string, func>& um, std::string& s){
    return um.find(s) != um.end(); 
}

每个重载函数的参数都不同。然而，从这一行(contains(opmap, q_front))我得到了错误：more than one instance of overloaded function "contains" matches the argument list.

作为参考，opmap是 类型std::unordered_map<char, op>，并且q_front是string。op在这种情况下只是我创建的结构 - 如果需要我可以发布，但我觉得在这种情况下这是不必要的。

我的问题是为什么我会收到这个错误，因为上面的函数调用应该唯一地调用第二个方法头：bool contains(std::unordered_map<char, op>& um, std::string& s){因为类型opmap匹配第一个参数，类型q_front是string。

更新：

完整的错误信息：

more than one instance of overloaded function "contains" matches the argument list: -- function "contains(std::unordered_map<char, op, std::hash<char>, std::equal_to<char>, std::allocator<std::pair<const char, op>>> &um, std::string s)" (declared at line 48 of "/Users/raleighclemens/Documents/Calc_cpp/calc.h") -- function "contains(std::unordered_map<char, op, std::hash<char>, std::equal_to<char>, std::allocator<std::pair<const char, op>>> &um, std::string &s)" (declared at line 49) -- argument types are: (std::unordered_map<char, op, std::hash<char>, std::equal_to<char>, std::allocator<std::pair<const char, op>>>, std::string)C/C++(308)

雷：

#include <iostream>
#include <string>
#include <functional>
#include <unordered_map>

#define LEFT 0
#define RIGHT 1
#define UNARY 0
#define BINARY 1

struct op{
    char symbol;
    uint8_t precedence;
    uint8_t assoc;
    uint8_t type;

    std::function<double (double, double)> ashley;

};

struct func{
    std::string symbol;
    uint8_t type;

    std::function<double (double, double)> ashley;
};

bool contains(std::unordered_map<char, op>& um, char c){
    return um.find(c) != um.end();
}

// returns true if string s is contained in unordered map um
bool contains(std::unordered_map<char, op>& um, std::string& s){
    return s.length() == 1 && contains(um, s[0]); 
}

// returns true if string s is contained in unordered map um
bool contains(std::unordered_map<std::string, func>& um, std::string& s){
    return um.find(s) != um.end(); 
}


int main(int argc, char** argv){

    std::unordered_map<char, op> opmap;
    op op1{'+', 2, LEFT, BINARY, [=] (double a, double b){return a + b;}};
    opmap.emplace('+', op1);

    std::cout << contains(opmap, "+");