printf() surprising console output

Question

I need to develop with C++ after a break of a few years. Anyway, I got problems following the tutorials I am currently reading. After writing the following code-snippet I expected to see 'Hello World' in my console, but I am only able to see 'Debug: StrangeChars'; What went wrong?

std::string myString("Hello World"); 
printf("* Debug: %s \n", myString);

Pick any other tutorial you might find. `printf` is not the C++ way. — arne, Sep 16 '13 at 10:19
You're lying to your compiler: `%s` format specifier says you'll be passing `char*`. — jrok, Sep 16 '13 at 10:19
Why is this downvoted? While this is clearly a beginner's problem, the question is perfectly precise and the answer is clear for anyone experienced. Not an intersting question for most, but well asked. — b.buchhold, Sep 16 '13 at 10:30
@b.buchhold I downvoted this question because of the duplicate. Very easy to find. — Dariusz, Sep 16 '13 at 11:34

score 16 · Accepted Answer · answered Sep 16 '13 at 10:20

16

printf relies on you passing the right arguments. %s requires a char *, you passed a std::string.

Try (C way)

char myString[] = "Hello World";
printf("* Debug: %s \n", myString);

Or (Hybrid C/C++ way)

std::string myString("Hello World"); 
printf("* Debug: %s \n", myString.c_str());

Or the C++ way:

std::string myString("Hello World"); 
std::cout << "* Debug " << myString << std::endl;

answered Sep 16 '13 at 10:20

Sergey L.

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+1 for the "C++ way". The other two options should be ignored, IMO. – Bret Kuhns Sep 16 '13 at 12:24
@BretKuhns The question originally also had the C tag which someone removed. – Sergey L. Sep 16 '13 at 13:15

score 3 · Answer 2 · answered Sep 16 '13 at 10:22

3

Try

printf("* Debug %s \n", myString.c_str());

or better still:

std::cout << "* Debug " << mystring << std::endl;

answered Sep 16 '13 at 10:22

graham.reeds

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score 1 · Answer 3 · edited Sep 16 '13 at 10:23

1

The string type referenced by printf's %s% is not the same as std::string.

In fact, when printf was specified, std::string did not even exist. In C++ you should use std::cout instead, which works as expected:

#include <iostream>
 [...]

std::string myString("Hello World"); 
std::cout << "* Debug: " << myString << " \n";

edited Sep 16 '13 at 10:23

Magnus Hoff

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answered Sep 16 '13 at 10:23

ComicSansMS

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score 0 · Answer 4 · answered Sep 16 '13 at 10:21

0

Use myString.c_str(); You're trying to output an object as a string. There's no conversion between std::string and char *

answered Sep 16 '13 at 10:21

Sebastian-Laurenţiu Plesciuc

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score 0 · Answer 5 · answered Sep 16 '13 at 10:28

printf is a C function and expects a c-string, a char* which is just a pointer to a place in memory where the contexts of the string reside. It assumes the string goes as far as the next '\0' byte and will print the contents of the memory from the address passed as pointer up to that point (the next '\0').

std::string is a class with several members. Apart from the actual string, it stores, for example, the current size. Besides, you did not pass a pointer to the string.

If you want to use std::string with printf, you'll have to call the c_str() method on the string object. This method returns the content of the string object as char* / c-string and should always be used when using string objects with c-function that expect c-strings.

printf("* Debug: %s \n", myString.c_str());

printf() surprising console output

5 Answers5