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+'

Solution 1

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.

Solution 2

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.

Solution 3

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

Comments

[Michael Krelin - hacker]

+1 but see my comment for the case like this.

[Bojangles]

Forgive my ignorance, but could you do std::string sql = /* String literals */?

[chris]

@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.

[Michael Krelin - hacker]

@chris, yes that's why I say for the case like this. In particular, for sql I'd prefer parametrized query ;)

[Michael Krelin - hacker]

How's throwing in heckuvalotta resources "better"?

[bames53]

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<std::string>(myFoo)

[chris]

@jacknad, I'd really consider accepting Michael's answer. It applies to your situation much better than this does since they're all literals.

[jacknad]

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.

[chris]

@jacknad, As long as those variables are std::strings, you can do something like string s = "abc" "def" + otherString + "ghi";

[jacknad]

Haven't touched C++ in a while. Between C, Java, C#, Vala, Python, etc., my head is spinning. But each has its own charm.

[Michael Krelin - hacker]

Yes, that + consider parametrized queries.

[Michael Krelin - hacker]

c++ is the most charming! ;-)

[mohaps]

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.

[Matthieu M.]

@mohaps: well, I would avoid using the pesky implementation-defined wide characters; best way not to have portable programs.

[mohaps]

@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.

[Matthieu M.]

@mohaps: well, programming only for the linux world, they are pretty rare since it's utf-8 everywhere :D

[chris]

There's still std::towstring by the way.