C++ cannot convert 'const char*' to 'std::string*'

Question

I have this code below and I'm getting the error upon compilation:

error: cannot convert 'const char*' to 'std::string*' for argument '1' to 'void sillyFunction(std::string*, int)'

#include <iostream>
#include <string>

using namespace std;
int counter = 0;

void sillyFunction(string * str, int cool=0);

int main(){
    sillyFunction("Cool");
    sillyFunction("Cooler", 1);
    return 0;
}

void sillyFunction(string * str, int cool){
    counter++;
    if (cool){
        for (int i=0; i<counter; i++) cout << *str << endl;
    } else {
        cout << *str << endl;
    }
}

Solution 1

Don't take your parameter in as a string * try just using a const string & instead

EDIT:

std::string and const char* are different types. the std::string already has a conversion from string literals (ex: "Cool") to the actual string object. So by passing in the string literal "Cool" you are in a sense passing in a std::string object, not a pointer to one.

The reason I chose to use a const string & is mostly from personal coding practices. This minimizes stack memory usage, and since you are passing in a constant string literal, there is no need for the parameter to be modify-able.

Also don't forget if you change from a string * that you no longer need to dereference it in your cout:

if (cool){
    for (int i=0; i<counter; i++) cout << str << endl;
} else {
    cout << str << endl;
}

Solution 2

change

void sillyFunction(string * str, int cool){
   counter++;
    if (cool){
        for (int i=0; i<counter; i++) cout << *str << endl;
    } else {
        cout << *str << endl;
    }
}

to

void sillyFunction(const char* str, int cool){
    counter++;
    if (cool){
        for (int i=0; i<counter; i++) cout << str << endl;
    } else {
        cout << str << endl;
    }
}

Solution 3

To explain what the problem actually is...

While the compiler will happily arrange for a char */C-string to be "converted" to a std::string via the appropriate std::string constructor, that's not what you've asked for.

You have asked for a pointer to an existing std::string object. By definition, what is passed to your function must be the address of an already existing std::string (or descendant) object.

You must understand pointers as a distinct type -- your function takes a pointer-to-std::string "object". While a std::string can be accessed via a pointer to std::string, the pointer itself is not a std::string, nor can it be "converted" to a std::string, nor can it be treated as a pointer-to-char (or vice-versa).

The easiest alternative is indeed a const reference to a std::string (const std::string &). const, in this case, because you're not doing anything to modify the string. If you were, that would be a different matter, and you'd have to consider carefully whether you intend for the caller to see your changes.

By doing this, you're saying you want a std::string object (remember, a reference to an object is that object, see C++ FAQ 8.5 in particular), which allows the compiler to invoke the appropriate constructor to create a std::string for you when the function is called with a char * (const or not).

At the same time, if someone passes you an actual std::string, the constructor is avoided and you get the same efficiency as if you had taken a pointer-to-std::string. Win-win.

Alternatively, of course, you can just take a plain std::string, but in that case you always get a copy of the string being passed in, whether it's a C-string or a std::string. Sometimes that's desirable, sometimes not. In your case, you don't do anything but print the string out, making the overhead unnecessary.

Solution 4

Should be

void sillyFunction(const string& str, int cool=0);

Solution 5

If you're going to use pointers for your functions you have to declare the strings somewhere. memory needs to be allocated. so either declare a variable or call new to create memory for the string.

int main(){
    string str = "Cool";
    string str2 = "Cooler";
    sillyFunction(&str);
    sillyFunction(&str2, 1);
    return 0;
}

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Comments

[karlphillip]

If you elaborate your answer and explain why he should do this I will upvote your answer.

[Pwnna]

Shouldn't it be const string &str?

[detunized]

@ultimatebuster, it's the same. It's just a matter of taste how to write that.