How do I convert a char array to integer/double/long type? [duplicate]

Either Boost.LexicalCast:

boost::lexical_cast<int>("42");

Or (C++11):

std::stoi("42");

Also, don't use char arrays unless it's interop. Use std::string instead. Also don't ever use ato* functions, even in C, they're broken as designed, as they can't signal errors properly.

Writing such a function yourself is a great exercise:

unsigned parse_int(const char * p)
{
    unsigned result = 0;
    unsigned digit;
    while ((digit = *p++ - '0') < 10)
    {
        result = result * 10 + digit;
    }    
    return result;
}

Of course, you should prefer existent library facilities in real world code.

Using C++ Streams

std::string hello("123"); 
std::stringstream str(hello); 
int x;  
str >> x;  
if (!str) 
{      
   // The conversion failed.      
} 

template<class In, class Out>
static Out lexical_cast(const In& inputValue)
        {
    Out result;

    std::stringstream stream(std::stringstream::in | std::stringstream::out);
    stream << inputValue;
    stream >> result;
    if (stream.fail() || !stream.eof()) {
        throw bad_cast("Cast failed");
    }

    return result;
}

using it:

int val = lexical_cast< std::string, int >( "123" );

Do you mean how to convert them to integers? You cannot convert an array of characters into a function on the language level - perhaps you can with some compiler specific inline assembler syntax. For doing a conversion into an integer you can use atoi

int i = atoi("123");`

Or strtol

long l = strtol("123", NULL, 10);