Cannot convert ‘std::string’ to ‘const char*

Question

What is wrong with this code?

string s = getString(); // Return string

if(!strcmp(s, "STRING")) {
    // Do something
}

While compiling, I am getting the error like:

error: cannot convert ‘std::string’ to ‘const char*’ for argument ‘1’ to ‘int strcmp(const char*, const char*)’|

You want if ( s == "STRING" ). strcmp is the const char* version. (You could use if (!strcmp(s.c_str(), "STRING"), but don't). — BoBTFish
– BoBTFish, Commented May 29, 2013 at 9:42
Did you look at the documentation for strcmp to see what arguments it accepts? Why do you even use strcmp like that when you already have an std::string that defines operator==? — Jon
– Jon, Commented May 29, 2013 at 9:42
@Rakkun: that's not good practice... the std::string type overloads comparison to a const char* to avoid creating a std::string temporary as your suggested code does.... — Tony Delroy
– Tony Delroy, Commented May 29, 2013 at 9:48
The beauty of C++: Where string == "foo" means exactly what it says. — stefan
– stefan, Commented May 29, 2013 at 9:51

awesoon · Accepted Answer · 2013-05-29 09:42:19Z

32

strcmp accepts const char* as argument. You can use c_str method:

if(!strcmp(s.c_str(),"STRING"))

Or just use overloaded operator== for std::string:

if(s == "STRING")

answered May 29, 2013 at 9:42

awesoon

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4 Comments

Haris Over a year ago

I am going to use if (s == "STRING") , but instead of this can I use if(getString() =="STRING")

Haris Over a year ago

My code is just like this pastebin.com/raw.php?i=rDcPApix And if do like this if (s == "STRING") I am getting the result.

awesoon Over a year ago

Your code works for me. Example

Haris Over a year ago

Hi thanks for your response. That was my coding problem, any way I solved it.

paxdiablo · Accepted Answer · 2013-05-29 09:42:38Z

12

You need to use s.c_str() to get the C string version of a std::string, along the lines of:

if (!strcmp (s.c_str(), "STRING")) ...

but I'm not sure why you wouldn't just use:

if (s == "STRING") ...

which is a lot more readable.

answered May 29, 2013 at 9:42

paxdiablo

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Comments

janm · Accepted Answer · 2013-05-29 09:43:54Z

5

You can use the c_str() method on std::string as in the other answers.

You can also just do this:

if (s == "STRING") { ... }

Which is clearer and doesn't pretend that you're writing C.

answered May 29, 2013 at 9:43

janm

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Comments

JBL · Accepted Answer · 2013-05-29 10:04:25Z

3

You must use the c_str() member function of std::string that gives you the underlying char array, if you want to keep the C way of comparing strings.

Otherwise, you should use the operator== which can test equality between strings and const char*.

edited May 29, 2013 at 10:04

answered May 29, 2013 at 9:42

JBL

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Comments

Thibel · Accepted Answer · 2013-05-29 09:45:03Z

1

You must use c_str() and it should solve your problem.

answered May 29, 2013 at 9:45

Thibel

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1 Comment

BoBTFish Over a year ago

That is one way, but it is the worse way.

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