C ++模板函数-编译器错误：“无法从const char *更改为int”

Question

I have a template function specified:

template<class T> void jsonParse(json_object* jobj, eParseWhat parseWhat, T &value) {
    ...

    if(parseWhat == PARSE_UID) {
        value = json_object_getInt(val);
    }
    if(parseWhar == PARSE_EMAIL) {
        value = json_object_getString(val);
    }
    ...
}

Now when I want to parse the uid of the json-object I call the method with an int :

json_object* obj = ...;
int uid = 0;
jsonParse(obj,PARSE_UID,uid);

But then a compile error occurs at the assignment in line:

value = json_object_getString(val);

Because of the call with an int the compiler thinks that the type of the variable value is int and json_object_getString(val) returns const char* . So the compiler says: can not convert from const char* to int .

Have you got any suggestion to solve this problem?

Answer 1

Why are you even using a template if you are going to do a switch statement of a bunch of if statements for each type? Your template instantiation will never compile if you want to treat every type as multiple types. This design is flawed but if you must then you can use specializations to accomplish something similar.

template<class T> void jsonParse(json_object* jobj, eParseWhat parseWhat, T &value) {
    static_assert(false); // not a handled type
}

template<> void jsonParse(json_object* jobj, eParseWhat parseWhat, int &value) {
    value = json_object_getInt(val);
}

template<> void jsonParse(json_object* jobj, eParseWhat parseWhat, std::string &value) {
    value = json_object_getString(val);
}

As GMan points out it is usually preferable to overload functions rather than specialize a function template. The equivalent using an overloaded function would be something like this:

void jsonParse(json_object* jobj, eParseWhat parseWhat, int &value) {
        value = json_object_getInt(val);
    }

void jsonParse(json_object* jobj, eParseWhat parseWhat, std::string &value) {
        value = json_object_getString(val);
    }

Answer 2

Didn't you intend to write:

    if(parseWhat == PARSE_UID) {
        value = json_object_getInt(val);
    }
    else if(parseWhar == PARSE_EMAIL) {
        value = json_object_getString(val);
    }

On a sidenote: template usage in your example is quite strange and confusing to me. I'd rather use some kind of Variant class for this (one example is boost::any , but you can easily implement your own - it's a matter of two hours).

Answer 3

Your code won't compile in any way, cause in that method T value can't be int and char* at the same time...

You try to cast differnt types (one is a int, on is a char) to a value (that isn't even defined) ;)

First you have to use:

json_object* obj = ...;
int uid = 0;

jsonParse<int>(obj,PARSE_UID,uid); //last parameter is int

or

jsonParse<string>(obj,PARSE_UID,whatever); //last parameter is string

But that don't work, as said before... you can't use a type in two diffrent ways in one template...

Sorry... :)