How to repair "invalid operands of types 'const char' to binary 'operator+'?

Question

Possible Duplicate:
How do I concatenate multiple C++ strings on one line?

According to this a C++ std::string is concatenated by using operator+. Why then does this code

using namespace std;
string sql = "create table m_table(" + 
    "path TEXT," +
    "quality REAL," +
    "found INTEGER);";

cause this error?

invalid operands of types 'const char [22]' and 'const char [17]' to binary 'operator+'

Those aren't C++ string objects you're trying to concatenate, but C strings. String literals are C strings. — Ben Voigt, Oct 05 '12 at 18:17

score 14 · Accepted Answer · answered Oct 05 '12 at 18:18

14

What chris said, but in this particular case you can do

string sql = "create table m_table("
    "path TEXT,"
    "quality REAL,"
    "found INTEGER);";

Which will concatenate strings at compile time.

answered Oct 05 '12 at 18:18

Michael Krelin - hacker

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Yes. This works for a set of const strings which I will use in this case but I probably also need what Chris said for the `insert into m_table` case which has variables and which I neglected to mention. – jacknad Oct 05 '12 at 18:39
@jacknad, As long as those variables are `std::string`s, you can do something like `string s = "abc" "def" + otherString + "ghi";` – chris Oct 05 '12 at 18:42
Haven't touched C++ in a while. Between C, Java, C#, Vala, Python, etc., my head is spinning. But each has its own charm. – jacknad Oct 05 '12 at 18:47
2

Yes, that + consider parametrized queries. – Michael Krelin - hacker Oct 05 '12 at 18:47
`c++` is the most charming! ;-) – Michael Krelin - hacker Oct 05 '12 at 18:48

score 12 · Answer 2 · answered Oct 05 '12 at 18:16

12

You need to explicitly convert it to a string to match the argument list:

string sql = std::string("create table m_table(") + 
"path TEXT," +
"quality REAL," +
"found INTEGER);";

Now the first one is a string matched up with a const char[N], which matches one of the operator+ overloads and returns a new std::string, which is used to repeat the process for the rest.

answered Oct 05 '12 at 18:16

chris

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+1 but see my comment for the case like this. – Michael Krelin - hacker Oct 05 '12 at 18:19
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Forgive my ignorance, but could you do `std::string sql = /* String literals */`? – Bojangles Oct 05 '12 at 18:19
@MichaelKrelin-hacker, Yes, very good point. It's often the case that there's a `char *` variable as one of the first two arguments, in which case you would need to explicitly convert one, but that doesn't apply here. – chris Oct 05 '12 at 18:21
@chris, yes that's why I say *for the case like this*. In particular, for `sql` I'd prefer parametrized query ;) – Michael Krelin - hacker Oct 05 '12 at 18:26
@jacknad, I'd really consider accepting Michael's answer. It applies to your situation much better than this does since they're all literals. – chris Oct 05 '12 at 18:37

score 1 · Answer 3 · answered Oct 05 '12 at 18:25

1

a better way is to use std::ostringstream

#include <sstream>

const std::string myFunc(const std::string& s1, const std::string& s2)
{
  std::ostringstream os;
  os<<s1<<" "<<s2;
  return os.str();
}

The advantage is that you can use the std::ostream << operator overloads to stringify non string values too

answered Oct 05 '12 at 18:25

mohaps

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How's throwing in heckuvalotta resources "better"? – Michael Krelin - hacker Oct 05 '12 at 18:27
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It is useful to be able to combine other types into a string, but I don't think there's any need to change simple string concatenation to get that. I think sticking with operator+ and just using `std::to_string()` or similar functions is a better way: `"hello" + std::to_string(50)`. lexical_cast even lets you do this with any type that has an ostream `operator<<`: `"Hello, " + boost::lexical_cast(myFoo)` – bames53 Oct 05 '12 at 18:30
Michael, it's better if you need to concatenate just more than strings and/or (by using std::wostringstream) need to handle wide char strings etc. the stringstream classes in STL were provided specifically for these type of usecases, extending the iostreams paradigm. the std::ostream semantics for formatting, precision, width etc. can also come into play here. – mohaps Oct 05 '12 at 20:11
@mohaps: well, I would avoid using the pesky implementation-defined wide characters; best way not to have portable programs. – Matthieu M. Oct 05 '12 at 20:13
@MatthieuM. Amen to that! :) But in a large enough codebase with pesky little legacy thirdparty libraries... it's better to plan ahead and allow for the dreaded wchar_t stuff. – mohaps Oct 05 '12 at 20:30
@mohaps: well, programming only for the linux world, they are pretty rare since it's utf-8 everywhere :D – Matthieu M. Oct 05 '12 at 20:33
There's still `std::towstring` by the way. – chris Oct 06 '12 at 16:05

How to repair "invalid operands of types 'const char' to binary 'operator+'?

3 Answers3