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

Posted on 2016-10-01
Medium Priority
Last Modified: 2016-10-03

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.

#include <string>

A(const char* filename);

void  AnotherFunc(const char* filename);


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?
Question by:LuckyLucks
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LVL 40

Expert Comment

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.
LVL 40

Expert Comment

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.
LVL 29

Assisted Solution

pepr earned 500 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

evilrix earned 1000 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.
LVL 29

Assisted Solution

pepr earned 500 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.
LVL 32

Assisted Solution

phoffric earned 500 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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