C++ create string of text and variables

Question

I'm trying to do something very simple and yet, after an hour of so of searching a I can't find a suitable answer so I must be missing something fairly obvious.

I'm trying to dynamically create filenames for use with ifstream. Whilst I understand various methods are available of doing this, I have settled on creating a std::string, and the using stringname.c_str to convert to const.

The problem is however that I need to create the string with a mix of variables and predefined text values. I'm getting compiler errors, so this must be a syntax issue.

Pseudo

std::string var = "sometext" + somevar + "sometext" + somevar;

Thanks!

if it's a syntax error you should post the actual code then we can tell you want the syntax error is (and why it's wrong) and you'll learn more than if we just give you the correct syntax. — twain249
– twain249, Commented Apr 18, 2012 at 22:53
std::string var = std::string("sometext") + somevar + "sometext" + somevar; — David Schwartz
– David Schwartz, Commented Nov 11, 2017 at 20:28
as @yury's solution shows you can also use printf style API with boost::format. — Trevor Boyd Smith
– Trevor Boyd Smith, Commented Apr 25, 2018 at 17:09

mgiuffrida · Accepted Answer · 2012-04-18 23:07:57Z

102

Have you considered using stringstreams?

#include <string>
#include <sstream>

std::ostringstream oss;
oss << "sometext" << somevar << "sometext" << somevar;
std::string var = oss.str();

edited Apr 18, 2012 at 23:07

answered Apr 18, 2012 at 22:54

mgiuffrida

3,6292 gold badges28 silver badges32 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

mip Over a year ago

Exactly + std::ostringstream would be sufficient.

Jack Farrow Over a year ago

Thanks @eli - This was the method which I had initially attempted without any luck, but is now working! Can you explain what ss.str() is doing at all?

David Schwartz Over a year ago

ss.str() is extracting the contents of the stringstream.

David Rinck · Accepted Answer · 2014-09-17 16:26:49Z

53

In C++11 you can use std::to_string:

std::string var = "sometext" + std::to_string(somevar) + "sometext" + std::to_string(somevar);

answered Sep 17, 2014 at 16:26

David Rinck

7,0565 gold badges48 silver badges65 bronze badges

Comments

David Schwartz · Accepted Answer · 2012-04-18 23:02:34Z

38

std::string var = "sometext" + somevar + "sometext" + somevar;

This doesn't work because the additions are performed left-to-right and "sometext" (the first one) is just a const char *. It has no operator+ to call. The simplest fix is this:

std::string var = std::string("sometext") + somevar + "sometext" + somevar;

Now, the first parameter in the left-to-right list of + operations is a std::string, which has an operator+(const char *). That operator produces a string, which makes the rest of the chain work.

You can also make all the operations be on var, which is a std::string and so has all the necessary operators:

var = "sometext";
var += somevar;
var += "sometext";
var += somevar;

answered Apr 18, 2012 at 23:02

David Schwartz

184k18 gold badges229 silver badges292 bronze badges

2 Comments

mip Over a year ago

...assuming that type of somevar is on the operator +(*type*) list.

David Schwartz Over a year ago

@doc Yep. Otherwise, you'll have to add some code around somevar to make it work. For example, if it's an int, you can use Boost::lexical_cast to make it a string. (Or, in that case, the stringstream solution might be better.)

alkino · Accepted Answer · 2019-12-04 09:39:57Z

15

The new way to do with c++20 is using format.

#include <format>

auto var = std::format("sometext {} sometext {}", somevar, somevar);

answered Dec 4, 2019 at 9:39

alkino

9338 silver badges15 bronze badges

Comments

QuantumBlack · Accepted Answer · 2012-04-18 22:57:21Z

7

You can also use sprintf:

char str[1024];
sprintf(str, "somtext %s sometext %s", somevar, somevar);

answered Apr 18, 2012 at 22:57

QuantumBlack

1,5561 gold badge13 silver badges28 bronze badges

5 Comments

David Schwartz Over a year ago

This isn't very safe though. The additional complexity of making it safe probably makes it not a very good choice.

mip Over a year ago

snprintf() would be a better choice. snprintf(str, 1024, "somtext %s sometext %s", somevar, somevar);

Yury Over a year ago

This isn't safe, but too handy in comparision with bulky std::string & ostringstream.

Vinícius Queiroz Over a year ago

@DavidSchwartz Why is it not safe? Do you mean that it is prone to cyberattacks? If yes, how so? Could you elaborate, please?

David Schwartz Over a year ago

@ViníciusQueiroz What if the formatted output exceeds 1023 characters and overflows str?

Yury · Accepted Answer · 2013-12-17 09:45:59Z

4

Comments

Autex · Accepted Answer · 2022-05-18 11:15:23Z

2

Since c++14 you can use the std::string operator""s from std::string_literals

#include <string>
using namespace std::string_literals;
std::string var = "sometext"s + somevar + "sometext"s + somevar;

The compiler will add a std::string in place of the literal, and as such you can use the "+" operator with it.

answered May 18, 2022 at 11:15

Autex

3772 silver badges12 bronze badges

1 Comment

Wormer Over a year ago

Looks greatly underrated. The new C++20 format thing looks like an overkill for such a simple task, while this solution is simple enough. I am just wondering are there any caveats with using std::string_literals?

William Banks · Accepted Answer · 2021-07-21 12:50:43Z

0

You could have something like:

#define Compose(...) ComposeFn({ __VA_ARGS__ })

std::string ComposeFn(std::initializer_list<std::string> strList) {
    std::ostringstream ss;
    for(std::string str : strList) {
        ss << str;
    }
    return ss.str();
}

And then use it like:

int errcode = 404;
std::cout << Compose("[ERROR]: (", errcode, ") doesn't exist") << std::endl;

The Compose macro is just to avoid using the curly brackets. You could also use a variadic function, but eh

answered Jul 21, 2021 at 12:50

William Banks

891 silver badge8 bronze badges

Collectives™ on Stack Overflow

C++ create string of text and variables

8 Answers 8

3 Comments

Comments

2 Comments

Comments

5 Comments

Comments

1 Comment

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

8 Answers 8

3 Comments

Comments

2 Comments

Comments

5 Comments

Comments

1 Comment

Comments

Linked

Related