CODEWITHSUNDEEP
c++stringcharstring-concatenation
Matthew Darnell
Matthew Darnell

Reputation: 1365

Why won't these strings concatenate in C++?

I have an example of two test programs I wrote in C++. The first one works fine, the first one errors. Please help me to explain what is going on here.

#include <iostream>
#include <string>
#include <stdint.h>
#include <stdlib.h>
#include <fstream>
using namespace std;

string randomStrGen(int length) {
static string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
string result;
result.resize(length);
for (int32_t i = 0; i < length; i++)
    result[i] = charset[rand() % charset.length()];
return result;
}

int main()
{
ofstream pConf;
pConf.open("test.txt");
pConf << "rpcuser=user\nrpcpassword=" 
     + randomStrGen(15)
     + "\nrpcport=14632"
     + "\nrpcallowip=127.0.0.1"
     + "\nport=14631"
     + "\ndaemon=1"
     + "\nserver=1"
     + "\naddnode=107.170.59.196";
pConf.close();
return 0;
}

It opens 'test.txt' and writes the data, no problem. This, however, does not:

#include <iostream>
#include <string>
#include <stdint.h>
#include <stdlib.h>
#include <fstream>
using namespace std;

string randomStrGen(int length) {
static string charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
string result;
result.resize(length);
for (int32_t i = 0; i < length; i++)
    result[i] = charset[rand() % charset.length()];
return result;
}

int main()
{
ofstream pConf;
pConf.open("test.txt");
pConf << "rpcuser=user\n"
     + "rpcpassword=" 
     + randomStrGen(15)
     + "\nrpcport=14632"
     + "\nrpcallowip=127.0.0.1"
     + "\nport=14631"
     + "\ndaemon=1"
     + "\nserver=1"
     + "\naddnode=107.170.59.196";
pConf.close();
return 0;
}

The only difference in the second program is that 'rpcpassword' has been moved to the next line.

matthew@matthew-Satellite-P845:~/Desktop$ g++ test.cpp 
test.cpp: In function ‘int main()’:
test.cpp:23:6: error: invalid operands of types ‘const char [14]’ and ‘const char [13]’ to binary ‘operator+’ 
  + "rpcpassword="

Upvotes: 0

Views: 224

Answers (2)

Boyd Stephen Smith Jr.
Boyd Stephen Smith Jr.

Reputation: 3202

"rpcuser=user\nrpcpassword=" + randomStrGen(15) + "\nrpcport=14632" groups like ("rpcuser=user\nrpcpassword=" + randomStrGen(15)) + "\nrpcport=14632". Here, + is always used with an argument of a class type, so you get std::string::operator+ after overload resolution.

"rpcuser=user\n" + "rpcpassword=" + randomStrGen(15) groups like ("rpcuser=user\n" + "rpcpassword=") + randomStrGen(15). In this case, the first + is used on two non-class types, so it's not overloaded and the language doesn't define + for two const char [] values. (I come from old C, so I was a bit it didn't just add them as char *s and give you a nice SIGSEGV at runtime.)

Upvotes: 4

Aasmund Eldhuset
Aasmund Eldhuset

Reputation: 37940

A string literal ("foo") in C++ is not of the type string; it is of the type const char[x], where x is the length of the string literal plus 1. And character arrays can't be joined with +. However, a character array can be joined with a string, and the result is a string, which may further be joined with character arrays. Therefore, "a" + functionThatReturnsString() + "b" works, but "a" + "b" does not. (Keep in mind that + is left associative; it is applied to the two leftmost operands first, and then to the result and the third operand, and so on.)

Upvotes: 5

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