Question

When using printf for string, I got :

string key = "123";
printf("Value is %s \n", key);

// output is: Value is < null >

But if I do it like this:

string key = "123";
printf("Value is: ");
printf(key.c_str());

then I get:

// output is: Value is 123

So what I did wrong with

printf %s

?

Thanks in advance.

Answer 1

std::string is a C++ class. So this doesn’t work because:

1. printf is a pure C function, which only knows how to deal with primitive types (int, double, char *, etc.).
2. printf is a variadic function. Passing a class type to a variadic function leads to undefined behaviour.¹

If you want to display a string, use std::cout:

std::cout << key << "\n";

If you simply must use printf, then this should work:

printf("%s\n", key.c_str());

c_str is a member function which returns a C-style string (i.e. a const char *). Bear in mind that it has some restrictions; you cannot modify or delete the string object in-between calling c_str() and using the result:

const char *p = key.c_str();
key = "something else";
printf("%s\n", p);  // Undefined behaviour

---

_{1. Or possibly implementation-defined, I don’t recall. Either way, it’s not going to end well.}