9

I'm having some trouble declaring a string variable. Code and the errors are here: http://pastebin.com/TEQCxpZd Any thoughts on what I'm doing wrong? Also, please keep it platform independent. Thanks!

#include <stdio.h>
#include <string>
using namespace std;

int main()
{
    string input; //Declare variable holding a string

    input = scanf; //Get input and assign it to variable
    printf(input); //Print text
    return 0;
}


Getting this from GCC:

main.cpp: In function ‘int main()’:
main.cpp:53:10: error: invalid conversion from ‘int (*)(const char*, ...)’ to ‘char’
main.cpp:53:10: error:   initializing argument 1 of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(_CharT) [with _CharT = char, _Traits = std::char_traits<char>, _Alloc = std::allocator<char>, std::basic_string<_CharT, _Traits, _Alloc> = std::basic_string<char>]’
main.cpp:54:14: error: cannot convert ‘std::string’ to ‘const char*’ for argument ‘1’ to ‘int printf(const char*, ...)’
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2
  • Possibly from Pascal where you can call a function without the (). Commented Jan 20, 2011 at 5:45
  • That's strange, C++ isn't Pascal. Commented Jan 20, 2011 at 17:50

3 Answers 3

Reset to default
11

You are mixing c++ and c I/O. In C++ this is,

#include <string>
#include <iostream>

int main(void)
{
   std::string input;
   std::cin >> input;
   std::cout << input;
   return 0;
 }
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4 Comments

@Mike: Have you included the iostream header? #include <iostream> Also, you should probably be using g++ to compile C++ programs, rather than gcc.
I did include iostream. The problem was in terminal I had to type g++ main.cpp -o hello_world instead of gcc. Thanks cody!
@Mike: You're welcome. Please don't take this as offensive, but it looks like you're a little confused between C and C++. They're actually different languages, even though one is derived largely from the other and they share a lot of the same syntax. If you're trying to learn the language, you need to pick one. As both answers indicate, there are very different idioms used in C++ (as compared to C) for things as simple as reading input. Everything but your title gives the impression you might be trying to use C in the first place, which explains why you write the code you did and used gcc.
Well I downloaded a C++ IDE that created hello world using stdio so I assumed it should compile but a few things happened; 1. I couldn't build projects in the IDE, 2. I didn't know much about GCC to know that it doesn't compile C and C++, I though G++ enable GCC to compile C++, 3. All my knowledge of C++ was on windows, and as minimal as it was, I assumed if iostream wasn't working it was a windows header. Now I'm fine and can continue using this book I got which was supposed to be platform independent C++ but wasn't working due to the fact I was using GCC not G++. Thanks again for your help!
3

I understand the question to be: How do you make a string declaration in C++? Here's a short program to demonstrate:

#include<iostream>
#include<cstdlib>
using namespace std;
int main()
{
    string your_name;
    cout << "Enter your name: ";
    cin >> your_name;
    cout << "Hi, " << your_name << "!\n";
    return 0;
}

So, include cstdlib at the start of your program. In practical terms, this means typing string instead of std::string, cout instead of std::cout and so on. The string variable itself (in the example, the string variable is your_name) is declared with string.

Let's say you've saved the program with the filename, 'str_example.cpp' To compile the program at the command line (in Linux):

g++ -o str_example str_example.cpp

This creates an executable object file called str_example (no file extension). And finally, assuming you're in the same directory as the program, to run it:

./str_example

The man page for g++ is extensive but not included by default. To install g++ documentation using the aptitude package manager:

sudo apt-get install gcc-7-doc

Note that the '7' refers to version 7; the current version at the time of writing. Hope that helps.

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Comments

2

cannot convert ‘std::string’ to ‘const char*’ for argument ‘1’ to ‘int printf(const char*, ...)’

input = scanf; //Get input and assign it to variable

You're trying to assign the function pointer to scanf to a string variable. You can't do that, which is why you're getting the first error. The proper syntax would be.

char buffer[BIG_ENOUGH_SIZE];
scanf("%*s", sizeof(buffer) - 1, buffer);
input = buffer;

But that's a very C-style way of doing things. The idiomatic way to read input in C++ is with std::cin >> input as Nathan suggested.

cannot convert ‘std::string’ to ‘const char*’ for argument ‘1’ to ‘int printf(const char*, ...)’

printf(input); //Print text

printf takes a const char* as its first argument, not a std::string. You can use .c_str() to convert to a C-style string. But never pass user input as the first argument to printf; the user can do nasty stuff by putting %'s in the string. If you insist on C-style output, the correct syntax is:

printf("%s", input.c_str());

But the C++-style alternative is std::cout << input;.

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2 Comments

Well I can't even compile Hello World properly. I think something is wrong with the compiler, whether the code is right or not, I know there is nothing wrong with #include <iostream> using namespace std; int main() { cout << "Hello World!" << endl; return 0; }
What seems more interesting here is that when building under MSVC the implicit cast from std::string to const char* succeeds but does not when compiled with GCC. I imagine this is because the MSVC implementation of std::string contains a cast operator for const char* whereas the GCC implementation does not?

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