Pass a string in C++

Quick probably obvious 开发者_JAVA百科question.

If I have:

void print(string input)
{
  cout << input << endl;
}

How do I call it like so:

print("Yo!");

It complains that I'm passing in char *, instead of std::string. Is there a way to typecast it, in the call? Instead of:

string send = "Yo!";
print(send);

---

You can write your function to take a const std::string&:

void print(const std::string& input)
{
    cout << input << endl;
}

or a const char*:

void print(const char* input)
{
    cout << input << endl;
}

Both ways allow you to call it like this:

print("Hello World!\n"); // A temporary is made
std::string someString = //...
print(someString); // No temporary is made

The second version does require c_str() to be called for std::strings:

print("Hello World!\n"); // No temporary is made
std::string someString = //...
print(someString.c_str()); // No temporary is made

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You should be able to call print("yo!") since there is a constructor for std::string which takes a const char*. These single argument constructors define implicit conversions from their aguments to their class type (unless the constructor is declared explicit which is not the case for std::string). Have you actually tried to compile this code?

void print(std::string input)
{
    cout << input << endl;
} 
int main()
{
    print("yo");
}

It compiles fine for me in GCC. However, if you declared print like this void print(std::string& input) then it would fail to compile since you can't bind a non-const reference to a temporary (the string would be a temporary constructed from "yo")

---

Well, std::string is a class, const char * is a pointer. Those are two different things. It's easy to get from string to a pointer (since it typically contains one that it can just return), but for the other way, you need to create an object of type std::string.

My recommendation: Functions that take constant strings and don't modify them should always take const char * as an argument. That way, they will always work - with string literals as well as with std::string (via an implicit c_str()).

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print(string ("Yo!"));

You need to make a (temporary) std::string object out of it.

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For easy stuff like printing, you can define a sort of function in your preprocessors like:

#define print(x) cout << x << endl

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The obvious way would be to call the function like this

print(string("Yo!"));

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Make it so that your function accepts a const std::string& instead of by-value. Not only does this avoid the copy and is therefore always preferable when accepting strings into functions, but it also enables the compiler to construct a temporary std::string from the char[] that you're giving it. :)

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Just cast it as a const char *.

print((const char *)"Yo!") will work fine.