C++ Linux Sockets adding variable in char *msg

Question

I'm trying to add a insert a variable to a send().

here is the code:

string num;

// + num + is the reason for the error. Any work around or suggestions?
char *msg = "GET /index.php?num=" + num + " HTTP/1.1\nhost: domain.com\n\n";

int len;
ssize_t bytes_sent;
len = strlen(msg);
bytes_sent = send(socketfd, msg, len, 0);

I'm getting the error:

test.cpp: In function âint main()â:
test.cpp:64: error: cannot convert âstd::basic_string<char, std::char_traits<char>, 
std::allocator<char> >â to âchar*â in initialization

--edit--

I tried to fix it with msg.c_str

cout << "send()ing message..."  << endl;
string msg = "GET /index.php?num=" + num + " HTTP/1.1\nhost: domain.com\n\n";   
int len;
ssize_t bytes_sent;
len = msg.lenght(); //updated to this and still gives me an error.
bytes_sent = send(socketfd, msg.c_str, len, 0);

Now it gives me the error:

error: argument of type âconst char* (std::basic_string<char, std::char_traits<char>, 
std::allocator<char> >::)()constâ does not match âconst char*â

There's a reason they made a string class and don't use char pointers and arrays directly anymore -- which is, stuff like chars + string + chars doesn't do the right thing. :P — cHao
– cHao, Commented Nov 25, 2012 at 21:45
error: âstruct std::stringâ has no member named âlenghtâ — user1553142
– user1553142, Commented Nov 25, 2012 at 21:58

cHao · Accepted Answer · 2012-11-25 21:59:07Z

3

"stuff" + num + "more stuff" doesn't do what you'd expect. Even if you were to convert str to a char pointer, and even if C++ let you add char pointers together, it'd end up doing the totally wrong thing.

(For reference, C++ doesn't let you add pointers together, because the result doesn't make any sense. Pointers are still just numbers, and adding two char pointers would basically amount to 0x59452448 + 0x10222250 or something like that, which would return you a pointer to some location that probably doesn't even exist yet...)

Try this:

string msg = string("GET /index.php?num=") + num + " HTTP/1.1\nhost: domain.com\n\n";
ssize_t bytes_sent = send(socketfd, msg.c_str(), msg.size(), 0);

edited Nov 25, 2012 at 21:59

answered Nov 25, 2012 at 21:50

cHao

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1 Comment

user1553142 Over a year ago

Hello, tried it. Got an error invalid conversion from âconst charâ to âcharâ

zch · Accepted Answer · 2012-11-25 21:45:37Z

1

std::string is not implicitly converted to char*. You need to use c_str.

answered Nov 25, 2012 at 21:45

zch

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Comments

Kylotan · Accepted Answer · 2012-11-25 21:48:38Z

1

Ideally, you should work entirely with strings (not char*) in your application, right up to the point where an API function requires a char*, and at that point you call c_str on the string to get a const char* for the function you're calling.

answered Nov 25, 2012 at 21:48

Kylotan

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David G · Accepted Answer · 2012-11-25 21:50:39Z

1

You're using num where it is uninitialized on the third line. Maybe you want:

std::string num;
std::string msg = "GET /index.php?num=" + num + " HTTP/1.1\nhost: domain.com\n\n";

edited Nov 25, 2012 at 21:50

answered Nov 25, 2012 at 21:43

David G

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7 Comments

user1553142 Over a year ago

Yes. Is there a way to do that? Inserting the variable between? I mean an alternative? I guess thats whats making the error.

David G Over a year ago

Why are you doing char + string + char in the first place?

user1553142 Over a year ago

I tried changing num into a char. but i get an error. error: invalid operands of types âconst char*â and âconst char [70]â to binary âoperator+â

David G Over a year ago

@user1553142 Use std::string for all your string handling. It eliminates problems like these.

user1553142 Over a year ago

I tried it, and still got an error. error: cannot convert âstd::stringâ to âconst voidâ for argument â2â to âssize_t send(int, const void, size_t, int)â

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