C++ how to add more strings into a method

Question

I have been working in Java since I started programming and decided to learn c++. What I wrote in Java looked like this:

showMessage("Hello world" + randomNumber);

And it showed text + integer or float or whatever. But it wont work in c++. Error message by xCode: Invalid operands to binary expression ('const char *' and 'float')

Cheers!

score 2 · Accepted Answer · answered Sep 29 '12 at 10:20

2

You can do a sprintf according to Anton, or to be more c++:

std::stringstream ss;
ss << "Hello, world " << randomNumber;
showmessage(ss.str());

(there's nothing wrong with sprintf, especially if you use snprintf instead).

answered Sep 29 '12 at 10:20

Michael Krelin - hacker

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score 1 · Answer 2 · answered Sep 29 '12 at 10:18

1

    ostringstream os;
    os<<"HelloWorld"<<randomnumber;
    string s;
    s = os.str();

string s now contains the string you want as a string object.

answered Sep 29 '12 at 10:18

user93353

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score 1 · Answer 3 · answered Sep 29 '12 at 10:18

Also you can use boost::lexical_cast to cast numbers into strings which is fastest method in most cases:

showMessage("Hello world" + boost::lexical_cast<std::string>(randomNumber));

showMessage declaration is

void showMessage(cosnt std::string& message)

score 1 · Answer 4 · 2012-09-29T10:25:02.617

1

Consider adding a new function that is able to convert several types to std::string:

template<typename ty>
string to_str(ty t)
{
   stringstream ss; ss << t;
   return ss.str();
}

Usage:

"Hello World " + to_str(123)

edited Sep 29 '12 at 10:25

answered Sep 29 '12 at 10:19

@MichaelKrelin-hacker It won't fail. Concatenating `char*` with `string` is overloaded already in `` – Sep 29 '12 at 10:23
I thought it's only for `string` being on the left. Maybe I've had this problem with old incomplete implementations, not sure. – Michael Krelin - hacker Sep 29 '12 at 10:25
Even through. It won't fail because it will be assigned to a string `string str = "hello World " + to_str(123);` – Sep 29 '12 at 10:30

score 0 · Answer 5 · answered Sep 29 '12 at 10:34

Define a class S. Then write

showMessage( S() << "Hello world" << randomNumber );

I've coded up the S class too many times for SO, and it's a good exercise to create it, hence, not providing the source code.

Note that you can reasonably call it StringWriter or something like that, and then just use a typedef for more concise code in function calls.

score 0 · Answer 6 · edited May 23 '17 at 10:24

0

I am not sure if c-style answer is fine, but I have already answer it here in a cocos2d-x question.

Trying to set up a CCLabelTTF with an integer as part of it's string in Cocos2d-X C++

edited May 23 '17 at 10:24

Community

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answered Sep 29 '12 at 11:39

m.ding

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score 0 · Answer 7 · answered Sep 29 '12 at 11:47

0

With C++11:

showMessage("Hello world" + std::to_string(randomNumber));

answered Sep 29 '12 at 11:47

Pete Becker

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score -1 · Answer 8 · answered Sep 29 '12 at 10:13

-1

you should print into the char* instead.

You could do something like

char* tempBuffer = new char[256];

sprintf_s(tempBuffer, 256, "Hello world %d", randomNumber);

showMessage(tempBuffer);

answered Sep 29 '12 at 10:13

SinisterMJ

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`printf` functionality is part of `C` and discouraged in `C++` – Dunes Sep 29 '12 at 10:14
As you can see he used sprintf which is safe – Denis Ermolin Sep 29 '12 at 10:15
There is nothing "safe" about `sprintf` in terms of C++. It's part of the whole `printf` family. – tenfour Sep 29 '12 at 10:16
All in all I am not aware of any "good" method in C++ / C. Boost has some nice stuff, but the STL I think does not have something easier than the above. – SinisterMJ Sep 29 '12 at 10:19
1

There's nothing unsafe about `printf` family, though, isn't `sprintf_s` windows function? – Michael Krelin - hacker Sep 29 '12 at 10:22
@AntonRoth, another thing is `%d` — according to error message `randomNumber` is float. – Michael Krelin - hacker Sep 29 '12 at 10:23
1

This will cause memory-leak if invoked repeatedly – Sep 29 '12 at 10:26
A better choice is `static char buf[256];` - C++ doesn't have a Garbage-Collector (GC) – Sep 29 '12 at 10:34
A better choice is `std::string` and forgetting that `sprintf` exists. – Sep 29 '12 at 10:42
There's also an option to use local `char buf[256]` if static is inappropriate. `std::string` is not *always* a better choice and surely unless accompanied with mental damage shouldn't be followed by forgetting that `sprintf` exists. The amount of downvoters is very depressing… – Michael Krelin - hacker Sep 29 '12 at 13:58

score -1 · Answer 9 · answered Sep 29 '12 at 10:26

In C++ the standard way to concatenate strings and primitives is to use stringstream. Which fulfils the same functionality (and a little bit more) as StringBuilder in Java (of course its API differs). However, if you are comfortable using cout then you should be fine.

eg.

#include <iostream>
#include <sstream>
#include <string>

using namespace std;

int main () {
  stringstream ss;
  ss << "Some string - " << 124; // build string    
  string str = ss.str(); // extract string

  cout << str << endl;
  return 0;
}

Quick reference for stringstream http://www.cplusplus.com/reference/iostream/stringstream/stringstream/

C++ how to add more strings into a method

9 Answers9