Solved

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

Posted on 2016-10-01
6
83 Views
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?
0
Comment
Question by:LuckyLucks
  • 3
  • 2
6 Comments
 
LVL 40

Expert Comment

by:evilrix
Comment Utility
Ate you sure that code represents the code generating the error? You've not used std::string anywhere in it that I can see.
0
 
LVL 40

Expert Comment

by:evilrix
Comment Utility
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.
0
 
LVL 28

Assisted Solution

by:pepr
pepr earned 125 total points
Comment Utility
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.
0
How to improve team productivity

Quip adds documents, spreadsheets, and tasklists to your Slack experience
- Elevate ideas to Quip docs
- Share Quip docs in Slack
- Get notified of changes to your docs
- Available on iOS/Android/Desktop/Web
- Online/Offline

 
LVL 40

Accepted Solution

by:
evilrix earned 250 total points
Comment Utility
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.
0
 
LVL 28

Assisted Solution

by:pepr
pepr earned 125 total points
Comment Utility
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.
0
 
LVL 32

Assisted Solution

by:phoffric
phoffric earned 125 total points
Comment Utility
>> 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.
0

Featured Post

How to improve team productivity

Quip adds documents, spreadsheets, and tasklists to your Slack experience
- Elevate ideas to Quip docs
- Share Quip docs in Slack
- Get notified of changes to your docs
- Available on iOS/Android/Desktop/Web
- Online/Offline

Join & Write a Comment

When writing generic code, using template meta-programming techniques, it is sometimes useful to know if a type is convertible to another type. A good example of when this might be is if you are writing diagnostic instrumentation for code to generat…
Introduction This article is the first in a series of articles about the C/C++ Visual Studio Express debugger.  It provides a quick start guide in using the debugger. Part 2 focuses on additional topics in breakpoints.  Lastly, Part 3 focuses on th…
The viewer will learn additional member functions of the vector class. Specifically, the capacity and swap member functions will be introduced.
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.

728 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

14 Experts available now in Live!

Get 1:1 Help Now