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C++ error cannot convert from std::string to const char *

Posted on 2016-10-01
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Last Modified: 2016-10-03
Hi

I am getting a cannot convert from 'std::string' to 'const char*' error error 2664 inside a constructor.

I haven't called the method yet from anywhere.  I have just set up the class .h and .cpp and I am running into this.  Note: i have #include <string> in the header file for the class.


A.h
#include <string>

public:
A(const char* filename);

private:
void  AnotherFunc(const char* filename);


---------------------

A.cpp
A::A(const char* filename){
   AnotherFunc(filename);            //line flagged
}

A::AnotherFunc(const char* filename){
}


I get an error at the line flagged , saying it cannot convert from std::string to const  char *. ideas why?
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Question by:LuckyLucks
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6 Comments
 
LVL 40

Expert Comment

by:evilrix
ID: 41824902
Ate you sure that code represents the code generating the error? You've not used std::string anywhere in it that I can see.
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LVL 40

Expert Comment

by:evilrix
ID: 41824903
If you are trying to construct A with a std::string, you can't. There is no implicit conversion from a string to a char pointer, const or otherwise.
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Assisted Solution

by:pepr
pepr earned 125 total points
ID: 41824972
It is more usual and more recommended not to use const char* arguments in new code.

In the past years, it was better to use const std::string& instead. In the latest years, the trend is even more radical, but I am not that experienced to talk about that.
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LVL 40

Accepted Solution

by:
evilrix earned 250 total points
ID: 41825001
I don't disagree with pepr; however, if you are only passing around a char pointer you have to be aware that this would involve the implicite construction of a temporary std::string, which is suboptimal for repeated calls. C++ is notorious for this and so you do need to be careful. Each temporary string involves a heap allocation internally (although some implementations of string to carry a small static buffer for small string optimisation. The point is, try and be consistent with your types and be aware of creating unnecessary temporaries.
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LVL 29

Assisted Solution

by:pepr
pepr earned 125 total points
ID: 41825007
To add to evilrix's comment... My recommendation is to switch to using std::string also outside he called methods. This way, passing the argument via a reference will not cause creating a temporary std::string object. The only exception would be cases when you really need to pass the char* pointer that you get somehow else (third party, not in your hands). Or, it will be the case when you pass a string literal.
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LVL 32

Assisted Solution

by:phoffric
phoffric earned 125 total points
ID: 41825016
>> If you are trying to construct A with a std::string, you can't.
Yeah, author's code doesn't show that, but that is what would give the error: A(std::string filename);

You could modify the constructor body:
A::A(std::string filename)
{
	AnotherFunc( filename.c_str() );
}

Open in new window

The const char * is now consistent in AnotherFunc().
If you are going with std::string, why not stick with it in your class.
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