Converting an std::string to a const char *

I've googled, but I can't find anything really comprehensive, so I decided I'd ask you guys..

How in the world do you convert an std::string to a const char* ?/


It seems like the ifstream doesn't like working with std::string's, only const char *'s..

Helllp :D

Use the string's c_str() member function.

[edit]There shouldn't be any problem using string containers with ifstream. Perhaps you should show a minimal code sample that is causing you problems.[/edit]

The getline version that works with strings is a free function

Code:

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std::string str;
std::getline(std::cin,str);

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Note also the the pointer returned by c_str() is only good so long as the std::string exists, and is unmodified. Thus it's fine for passing to functions, but don't try returning a c_str() from a local string and so forth.

Here is the broken bit of code

Code:

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bool MS3DModel::Load_MS3D_Model(std::string Filename)
{
        ifstream inputFile( Filename, ios::in | ios::binary | ios::nocreate );
        if ( inputFile.fail())
                return false;        // "Couldn't open the model file."

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This spawns this error

Code:

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\\student\courses\cedar_cliff\computer\senior project\auditorium\resource_manager\ms3d.cpp(142) : error C2664: '__thiscall ifstream::ifstream(const char *,int,int)' : cannot convert parameter 1 from 'class std::basic_string<char,struct std::char_tra
its<char>,class std::allocator<char> >' to 'const char *'

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ifstream input file only takes a const char * for its first parameter

EDIT:

Fixed it with c_str()

Code:

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bool MS3DModel::Load_MS3D_Model(std::string Filename)
{
        const char* cstr = Filename.c_str();

        ifstream inputFile( cstr, ios::in | ios::binary | ios::nocreate );
        if ( inputFile.fail())
                return false;        // "Couldn't open the model file."

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Code:

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bool MS3DModel::Load_MS3D_Model(std::string Filename)

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Consider passing strings by reference.

Code:

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bool MS3DModel::Load_MS3D_Model(std::string &Filename)

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In this case const reference would be preferred.

so instead of passing it as a value pass it as a reference? and then still perform the conversion?

Can you explain why? :)

Technically you can save a further step and directly pass it as:

Code:

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bool MS3DModel::Load_MS3D_Model( filename.c_str() );

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which passes as a const char *.

Quote:

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Originally Posted by dpro

Technically you can save a further step and directly pass it as:

Code:

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bool MS3DModel::Load_MS3D_Model( filename.c_str() );

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which passes as a const char *.

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that doesn't really gain you anything. on most platforms a pointer and a reference produce equivalent code. so all you've really done is to deprive yourself of any std::string functionality (append, finding, etc) within Load_MS3D_Model

Fortunately the only thing I *might* want Load_MS3D_Models to do is assert that the file extension is correct.. That I can do with a char array though can't I?

Actually I recind my earlier statement, in thinking it through it is better to pass it by const reference than to convert and send the parameter. You gain much greater functionality through the code to leave it as a const ref. So listen to other people and forget I said anything about converting it as a const char *

Quote:

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Originally Posted by dpro

While that is true, I suppose it depends on how you look at it. I suppose if you want to change the parameter, it would be fine, but to me it seems simpler that way, but you are free to feel differently of course.

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even if you don't want to change the parameter, std::string is still useful. what if you want to check that filename ends in ".md3" or similar? you'd have to use one of the c string functions (can't even remember which one). why mix APIs when you don't have to?

Basically, stay as high-level as you can for as long as you can.

Lol don't worry about it chaos, I know I was just being narrow-sighted, I edited my post to reflect this. I'm a cranky man who is tired and forgot his c++ programming pills today.

Well, you can't exactly see the top level of my implementation..

At the top, while we're still working with an std::string, I do a file extension check to decide which loader function we will use..

then according to what the file extension is == to we pick the proper loader function and let it fly from there...

Still its better to stay and keep your options open as a std::string, rather than limit them by transferring over to a c-style string. More flexible that way.

Well, I'd like to keep it that way if I could.

But the ifsteam inputfile function chokes if I try to give it an std::string

that's the "as long as you can" bit :D

use std::string like this

Code:

---

bool MS3DModel::Load_MS3D_Model(const std::string &Filename)
{
        ifstream inputFile( Filename.c_str(), ios::in | ios::binary | ios::nocreate );
// more code here!
}

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that should work.

Thats what I had in there about 10 posts ago :D

With the assert you mentioned:

Code:

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bool MS3DModel::Load_MS3D_Model(const std::string &Filename)
{
        assert(Filename.length() > 5 && Filename.substr(Filename.length() - 5) == ".ms3d");
        ifstream inputFile( Filename.c_str(), ios::in | ios::binary | ios::nocreate );
// more code here!
}

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Quote:

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Originally Posted by Shamino

Thats what I had in there about 10 posts ago :D

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Except you were creating a temporary.

And passing by value.

Yeah yeah :D, checkout the full system WIP, I posted about it..