How to use printf with std::string

c++, namespaces, printf, std, string

Solution

C++23 Update

We now finally have `std::print` as a way to use `std::format` for output directly:

#include <print>
#include <string>

int main() {
    // ...
    std::print("Follow this command: {}", myString);
    // ...
}

This combines the best of both approaches.

Original Answer

It's compiling because `printf` isn't type safe, since it uses variable arguments in the C sense1. `printf` has no option for `std::string`, only a C-style string. Using something else in place of what it expects definitely won't give you the results you want. It's actually undefined behaviour, so anything at all could happen.

The easiest way to fix this, since you're using C++, is printing it normally with `std::cout`, since `std::string` supports that through operator overloading:

std::cout << "Follow this command: " << myString;

If, for some reason, you need to extract the C-style string, you can use the `c_str()` method of `std::string` to get a `const char *` that is null-terminated. Using your example:

#include <iostream>
#include <string>
#include <stdio.h>

int main()
{
    using namespace std;

    string myString = "Press ENTER to quit program!";
    cout << "Come up and C++ me some time." << endl;
    printf("Follow this command: %s", myString.c_str()); //note the use of c_str
    cin.get();

    return 0;
}

If you want a function that is like `printf`, but type safe, look into variadic templates (C++11, supported on all major compilers as of MSVC12). You can find an example of one here. There's nothing I know of implemented like that in the standard library, but there might be in Boost, specifically `boost::format`.

[1]: This means that you can pass any number of arguments, but the function relies on you to tell it the number and types of those arguments. In the case of `printf`, that means a string with encoded type information like `%d` meaning `int`. If you lie about the type or number, the function has no standard way of knowing, although some compilers have the ability to check and give warnings when you lie.

Problem

My understanding is that `string` is a member of the `std` namespace, so why does the following occur? ``` #include <iostream> int main() { using namespace std; string myString = "Press ENTER to quit program!"; cout << "Come up and C++ me some time." << endl; printf("Follow this command: %s", myString); cin.get(); return 0; } ``` Each time the program runs, `myString` prints a seemingly random string of 3 characters, such as in the output above.

Original source

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