I suspect the char* is not written to, it is only non const because it is a legacy API. If so, your problem is probably that the std::string has fallen out of scope or been modified between the point where you call c_str and where it is used in the guts of the API.
What's the difference between char * and const_cast<char*>(string.c_str())
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30-11-2021 - |
سؤال
I use an external library to deal with udp (OSC) communication between 2 apps. To format the messages that will be sent, the library is expecting a char* but I get a string from the UI that I have to convert.
While I was dealing with other parts of my code the udp part was hard coded like that :
char* endofMess = "from setEndMess";
and was working fine. I thought it would be easy to get it working with my strings and wrote :
std::string s = "from setEndMess";
char* endofMess = const_cast<char*>(s.c_str());
but unlike for the first example where I was receiving the message correctly formatted, I now receive only gibberish characters. Does somebody know where it can come from?
Thanks!
Matthieu
EDIT : the code I use : The method to send the message each time OSCVal will change :
void osc500::testOSC(int identifier, float OSCval)
{
UdpTransmitSocket transmitSocket( IpEndpointName( destIP, port ) );
char buffer[1024];
osc::OutboundPacketStream p( buffer, 1024 );
p << osc::BeginBundleImmediate
<< osc::BeginMessage( endofMess )
<< OSCval << osc::EndMessage
<< osc::EndBundle;
transmitSocket.Send( p.Data(), p.Size() );
}
And if I have to change the OSC pattern I call this one :
void osc500::setEndMess(String endpattern){
// endofMess = "from setEndMess"; //OK works fine each time it's called
//1st try :
//std::string s = "from setEndMess";
//endofMess = const_cast<char*>(s.c_str()); //gibberish
//2nd try :
//std::string s = "from setEndMess";
//endofMess = &s[0]; //gibberish
//3rd & 4th tries :
//char s[4] = {'t','e','s','t'};
//char s[5] = {'t','e','s','t','\0'};
//endofMess = s; //gibberish
}
المحلول 2
نصائح أخرى
c_str()
is for read-only access of std::string
.
If you want to write to a string through pointers, then use either...
- an array (or vector) of
char
instead ofstd::string
char* buf = &str[0];
- point directly to the first character of a string
Trick (2) is guaranteed to work under C++11; in practice it works in C++03 too but that relies on compiler implementation of std::string
having contignuos storage.
(And whatever you do, keep an eye on the buffer length.)
If you want to modify the std::string
content, I think you should use &s[0]
(making sure that the string is big enough).
std::string s = "abcdef...";
char* ptr = &s[0];
e.g. (tested with MSVC10):
#include <iostream>
#include <ostream>
#include <string>
using namespace std;
void f(char* ptr, size_t count)
{
for (size_t i = 0; i < count; i++)
ptr[i] = 'x';
}
int main()
{
string s = "abcdef";
cout << s << endl;
f(&s[0], 3);
cout << s << endl;
}
Output:
abcdef
xxxdef