I'm trying to get std::string from attribute's value with TinyXml.
The only thing I can get is a const char * val, and I can't find any way to convert from const char * to a std::string.
so two possible answers to that:
1. How to get a string of an attribute with TinyXml?
2. How to convert const char * val to string val.
this is the code I have now:
TiXmlElement* data;
data->Attribute("some_name"); // return const char * which seems like unconvertible.
After googeling, I tried this:
char * not_const= const_cast<char *> (data->Attribute("some_name"));
There are no errors in the code itself, but after compiling and running I get exceptions.
std::string has a constructor that takes char const*. You don't need a char* for that.
std::string str = data->Attribute("some_name");
However, be aware that std::string doesn't like NULL values, so don't give it any.
Related
I am trying to convert a char pointer to a string pointer but I am not sure if I am doing it correctly. I just wanted to post what I was trying and see if it was correct.
For context, I have a char * called ent->d_name and I need that to become a string *.
This is what I have been doing:
std::string arg = std::string(ent->d_name);
std::string * arg_p = &arg;
Command::_currentCommand->insertArgument(arg_p);
The insert command function takes a string pointer.
You could use
std::string *arg_p = new std::string(ent->d_name);
It will create a memory leak unless you delete each string after use, but apart from that it will work.
I am trying to make a new const char* b by adding a new string "hello" to original const char* a:
const char* a = some_code_here;
const char* b = (a + "_hello").c_str();
And the error I get is:
error: invalid operands of types const char* and const char [6] to binary operator+
Is there anything wrong I am doing?
Switch to strings, that is std::string.
Repeat after me, forget about using char or C-style strings.
As you have demonstrated, this is one of many issues.
Did I say switch to std::string?
Your char * is a pointer. Nothing more, nothing less, a pointer. A pointer to a single char; not a structure. The char data type doesn't have methods.
Switch to std::string.
You can add (concatenate) std::string.
Switch to std::string.
The std::string has the c_str() method. Don't use unless you understand the consequences; completely.
You can't arbitrarily add const char* in C++. These objects are just pointers to a contiguous section of memory, so adding them doesn't make sense. Instead, you should use the std::string class:
std::string a = "something";
std::string b = a + "_hello";
I know there are a lot of questions like this out there on StackOverflow, but I haven't been able to find any that help resolve my case. Whenever I try to do something like this:
// str = some string or char array
// some magic to get around fpermissive errors
stringstream convert;
convert << str;
// capture the stream's temporary string
const string store = convert.str();
// get a manageable array
const char* temp = store.c_str();
and then try to do something like atoi(temp[0]), I keep getting the classic conversion error that char couldn't be converted to const char. In the documentation for atoi and many other functions, const char is a required parameter. How can a char be sent in if there's only a const one? Does retrieving a char at a specific position auto-cast to char?
I'm not sure if this is what is causing the error, but atoi takes as its parameter not a char, but the pointer to one. So instead of
atoi(temp[0])
try this
atoi(&temp[0])
as that is a pointer.
I am converting a project written in C++ for windows. Everything is going fine (meaning I clearly see what needs to be changed to make things proper C++) until I hit this, which is my own little routine to find a keyword in along string of keyword=value pairs:
bool GetParameter(const char * haystack, const char *needle) {
char *search, *start;
int len;
len = strlen(needle) + 4; // make my own copy so I can upper case it...
search = (char *) calloc(1,len);
if (search == NULL) return false;
strcpy(search,needle);
strupr(search);
strcat(search,"="); // now it is 'KEYWORD='
start = strstr(haystack,search); <---- ERROR from compiler
g++ is telling me "Invalid conversion from const char * to char * "
(the precise location of the complaint is the argument variable 'search' )
But it would appear that g++ is dyslexic. Because I am actually going the other way. I am passing in a char * to a const char *
(so the conversion is "from char * to const char *" )
The strstr prototype is char * strstr(const char *, const char *)
There is no danger here. Nothing in any const char * is being modified.
Why is it telling me this?
What can I do to fix it?
Thanks for any help.
The background to the problem is that C defines the function strstr as:
char* strstr(const char*, const char*);
This is because C doesn't allow overloaded functions, so to allow you to use strstr with both const and non-const strings it accepts const strings and returns non-const. This introduces a weakness in C's already fragile type-system, because it removes const-ness from a string. It is the C programmer's job to not attempt to write via a pointer returned from strstr if you pased in non-modifiable strings.
In C++ the function is replaced by a pair of overloaded functions, the standard says:
7. The function signature strstr(const char*, const char*) shall be replaced by the two declarations:
const char* strstr(const char* s1, const char* s2);
char* strstr( char* s1, const char* s2);
both of which shall have the same behavior as the original declaration.
This is type-safe, if you pass in a const string you get back a const string. Your code passes in a const string, so G++ is following the standard by returning a const string. You get what you asked for.
Your code compiles on Windows because apparently the standard library you were using on Windows doesn't provide the overloads and only provides the C version. That allows you to pass in const strings and get back a non-const string. G++ provides the C++ versions, as required by the standard. The error is telling you that you're trying to convert the const return value to a non-const char*. The solution is the assign the return value to a const char* instead, which is portable and compiles everywhere.
Error is not regarding the arguments to stsrtr. Compiler is complaining about the conversion of the 'const char *' returned by strstr. You can't assign it to *start which is just char *
You can try one of these:
const char *start;
or
string start(strstr(haystack,search));
Although declaring start as const char* might suffice, what seems more appropriate to me is to use std::string objects instead:
#include <string>
#include <cctype>
#include <algorithm>
bool GetParameter(const char * haystack, const char *needle) {
std::string hstr(haystack), nstr(needle);
std::transform(nstr.begin(), nstr.end(),nstr.begin(), ::toupper);
nstr += "=";
std::size_t found = hstr.find(nstr);
if (found != std::string::npos) {
... // "NEEDLE=" found
}
else {
...
}
...
}
The conversion it is complaining about is from strstr(...) to start. Change the declaration of start to const char* start;
you can use such like:
start = const_cast<char *>(strstr( haystack, static_cast<const char *>(search) ));
I am not able to concat two const char*.
I do the following:
const char* p = new char[strlen(metadata.getRoot())+strlen(metadata.getPath())];
strcat(const_cast<char*>(p),metadata.getRoot());
strcat(const_cast<char*>(p),metadata.getPath());
strcpy(const_cast<char*>(args2->fileOrFolderPath),p);
function(args2->fileOrFolderPath);
Now when I print the variable args2->fileOrFolderPath on the console then the correct output appears... But when I call a method with the variable as parameter, and work with the variable then I got a segmentation fault. What is the problem?
I did not declare them like this but i know they have this information
So, I have this:
const char* ruta1 = "C:\\Users\\Deivid\\Desktop\\";
const char* ruta2 = "lenaGris.xls";
Then I used this for concatenation:
char * RutaFinal = new char[strlen(ruta1) + strlen(ruta2) + 1];
strcpy(RutaFinal, ruta1);
strcat(RutaFinal, ruta2);
printf(RutaFinal);
This worked for me.
I would prefer using std::string for this, but if you like char* and the str... functions, at least initialize p before using strcat:
*p = 0;
BTW:
using std::string, this would be:
std::string p = std::string(metadata.getRoot()) + metadata.getPath();
strcpy(const_cast<char*>(args2->fileOrFolderPath), p.c_str());
function(args2->fileOrFolderPath);
And you don't have to deallocate p somewhere.
1.
const char* p=new char[strlen(metadata.getRoot())+strlen(metadata.getPath())+1];
the length plus 1 to store '\0'.
2.
strcpy(const_cast<char*>(args2->fileOrFolderPath),p);
You can not guarantee args2->fileOrFolderPath 's length is longger than strlen(p).
This works well
#include <iostream>
using namespace std;
void foo(const char*s){
cout<<s<<endl;
}
int main(int argc,char*argv[]){
const char* s1 = "hello ";
const char* s2 = "world!";
const char* p = new char [strlen(s1)+strlen(s2)+1];
const char* s = new char [strlen(s1)+strlen(s2)+1];
strcat(const_cast<char*>(p),s1);
strcat(const_cast<char*>(p),s2);
strcpy(const_cast<char*>(s),p);
cout<<s<<endl;
foo(s);
return 0;
}
You have char pointers, pointing to char constants which can't be modified . What you can do is to copy your const char array to some char array and do like this to concate const strings :
char result[MAX];
strcpy(result,some_char_array); // copy to result array
strcat(result,another_char_array); // concat to result array
I believe you need to include space for the null terminator, and the first parameter to strcat shouldn't be const as you're trying to modify the memory being pointed to.
You want to do something like this:
char * str1 = "Hello, ";
char * str2 = "World!\n";
char * buffer = malloc(strlen(str1) + strlen(str2) + 1);
strcpy(buffer, str1);
strcat(buffer, str2);
printf(buffer);
Which prints out "Hello, World!" as expected.
As for the error you're seeing when using a parameter, I've wrote some tests to see why it doesn't break when using a const local variable. While compiling using a const char * for the pointer I'm using as the target I get this warning:
./strings.c:10: warning: passing argument 1 of ‘strcat’ discards qualifiers from pointer target type
As it states, const is discarded and it works as expected. However, if I pass a parameter which is a const char * pointer, then I get a bus error when trying to modify the buffer it writes to. I suspect what is happening is that it ignores the const on the argument, but it can't then modify the buffer because it's defined as const elsewhere in the code.