I have std::string variable. And I need to put some bytes from array of unsigned chars to it. I know the first byte and the the legth.
I can use the std::string::assign function. I've done it.
But I want to solve that issue in a right way using the memcpy function.
std::string newString;
memcpy(&newString, &bytes[startIndex], length);
I know it is wrong. I have researched and found some ideas using std::vector.
Please help me to find the most elegant solution for this issue.
Since we're just constructing the string, there is a std::string constructor that takes two iterators:
template< class InputIt >
basic_string( InputIt first, InputIt last,
const Allocator& alloc = Allocator() );
which we can provide:
std::string newString(&bytes[startIndex], &bytes[startIndex] + length);
If we're not constructing the string and are instead assigning to an existing one, you should still prefer to use assign(). That is precisely what that function is for:
oldString.assign(&bytes[startIndex], &bytes[startIndex] + length);
But if you really insist on memcpy() for some reason, then you need to make sure that the string actually has sufficient data to be copied into. And then just copy into it using &str[0] as the destination address†:
oldString.resize(length); // make sure we have enough space!
memcpy(&oldString[0], &bytes[startIndex], length);
†Pre-C++11 there is technically no guarantee that strings are stored in memory contiguously, though in practice this was done anyway.
You need to set the size of the string so that there will be a properly sized buffer to receive the data, and cast the constness out of the pointer you get from data()
std::string newString;
newString.resize(length);
memcpy((char*)newString.data(), &bytes[startIndex], length);
of course all of this is in the realm of undefined behavior, but pretty standard non the less.
It's a hack and as you said wrong way but it is possible since STL guarantees that std::string has contiguous storage:
std::string str(32, '\0');
std::strcpy(const_cast<char*>(str.data()), "REALLY DUDE, IT'S ILLEGAL WAY");
Of course, you can use std::memcpy in the same way (I used strcpy just to copy null-terminated string)...
In your case:
str.resize(length);
memcpy(const_cast<char*>(str.data()), bytes + startIndex, length);
Related
I have been working with C++ strings and trying to load char * strings into std::string by using C functions such as strcpy(). Since strcpy() takes char * as a parameter, I have to cast it which goes something like this:
std::string destination;
unsigned char *source;
strcpy((char*)destination.c_str(), (char*)source);
The code works fine and when I run the program in a debugger, the value of *source is stored in destination, but for some odd reason it won't print out with the statement
std::cout << destination;
I noticed that if I use
std::cout << destination.c_str();
The value prints out correctly and all is well. Why does this happen? Is there a better method of copying an unsigned char* or char* into a std::string (stringstreams?) This seems to only happen when I specify the string as foo.c_str() in a copying operation.
Edit: To answer the question "why would you do this?", I am using strcpy() as a plain example. There are other times that it's more complex than assignment. For example, having to copy only X amount of string A into string B using strncpy() or passing a std::string to a function from a C library that takes a char * as a parameter for a buffer.
Here's what you want
std::string destination = source;
What you're doing is wrong on so many levels... you're writing over the inner representation of a std::string... I mean... not cool man... it's much more complex than that, arrays being resized, read-only memory... the works.
This is not a good idea at all for two reasons:
destination.c_str() is a const pointer and casting away it's const and writing to it is undefined behavior.
You haven't set the size of the string, meaning that it won't even necessealy have a large enough buffer to hold the string which is likely to cause an access violation.
std::string has a constructor which allows it to be constructed from a char* so simply write:
std::string destination = source
Well what you are doing is undefined behavior. Your c_str() returns a const char * and is not meant to be assigned to. Why not use the defined constructor or assignment operator.
std::string defines an implicit conversion from const char* to std::string... so use that.
You decided to cast away an error as c_str() returns a const char*, i.e., it does not allow for writing to its underlying buffer. You did everything you could to get around that and it didn't work (you shouldn't be surprised at this).
c_str() returns a const char* for good reason. You have no idea if this pointer points to the string's underlying buffer. You have no idea if this pointer points to a memory block large enough to hold your new string. The library is using its interface to tell you exactly how the return value of c_str() should be used and you're ignoring that completely.
Do not do what you are doing!!!
I repeat!
DO NOT DO WHAT YOU ARE DOING!!!
That it seems to sort of work when you do some weird things is a consequence of how the string class was implemented. You are almost certainly writing in memory you shouldn't be and a bunch of other bogus stuff.
When you need to interact with a C function that writes to a buffer there's two basic methods:
std::string read_from_sock(int sock) {
char buffer[1024] = "";
int recv = read(sock, buffer, 1024);
if (recv > 0) {
return std::string(buffer, buffer + recv);
}
return std::string();
}
Or you might try the peek method:
std::string read_from_sock(int sock) {
int recv = read(sock, 0, 0, MSG_PEEK);
if (recv > 0) {
std::vector<char> buf(recv);
recv = read(sock, &buf[0], recv, 0);
return std::string(buf.begin(), buf.end());
}
return std::string();
}
Of course, these are not very robust versions...but they illustrate the point.
First you should note that the value returned by c_str is a const char* and must not be modified. Actually it even does not have to point to the internal buffer of string.
In response to your edit:
having to copy only X amount of string A into string B using strncpy()
If string A is a char array, and string B is std::string, and strlen(A) >= X, then you can do this:
B.assign(A, A + X);
passing a std::string to a function from a C library that takes a char
* as a parameter for a buffer
If the parameter is actually const char *, you can use c_str() for that. But if it is just plain char *, and you are using a C++11 compliant compiler, then you can do the following:
c_function(&B[0]);
However, you need to ensure that there is room in the string for the data(same as if you were using a plain c-string), which you can do with a call to the resize() function. If the function writes an unspecified amount of characters to the string as a null-terminated c-string, then you will probably want to truncate the string afterward, like this:
B.resize(B.find('\0'));
The reason you can safely do this in a C++11 compiler and not a C++03 compiler is that in C++03, strings were not guaranteed by the standard to be contiguous, but in C++11, they are. If you want the guarantee in C++03, then you can use std::vector<char> instead.
In a recent question, I learned that there are situations where you just gotta pass a char* instead of a std::string. I really like string, and for situations where I just need to pass an immutable string, it works fine to use .c_str(). The way I see it, it's a good idea to take advantage of the string class for its ease of manipulation. However, for functions that require an input, I end up doing something like this:
std::string str;
char* cstr = new char[500]; // I figure dynamic allocation is a good idea just
getstr(cstr); // in case I want the user to input the limit or
str = cstr; // something. Not sure if it matters.
delete[] cstr;
printw(str.c_str());
Obviously, this isn't so, uh, straightforward. Now, I'm pretty new to C++ so I can't really see the forest for the trees. In a situation like this, where every input is going to have to get converted to a C string and back to take advantage of string's helpful methods, is it just a better idea to man up and get used to C-style string manipulation? Is this kind of constant back-and-forth conversion too stupid to deal with?
In the example you give, you can generally read a line into a std::string using the std::getline function: http://www.cplusplus.com/reference/string/getline/
Of course this doesn't do everything that a curses library does. If you need a non-const char* so that some C function can read into it, you can use a vector<char>. You can create a vector<char> from a string, and vice-versa:
std::string a("hello, world");
std::vector<char> b(a.begin(), a.end());
// if we want a string consisting of every byte in the vector
std::string c(b.begin(), b.end());
// if we want a string only up to a NUL terminator in the vector
b.push_back(0);
std::string d(&b[0]);
So your example becomes:
std::vector<char> cstr(500);
getnstr(&cstr[0], 500);
printw(&cstr[0]);
Most std::string::c_str() implementations (if not all of them) simply return a pointer to an internal buffer. No overhead whatsoever.
Beware however that c_str() returns a const char*, not a char*. And that the pointer will become invalid after the function call. So you cannot use it if the function does anything like writing back into the passed string or makes a copy of the pointer.
I want to convert a C-style string into a byte-vector.
A working solution would be converting each character manually and pushing it on the vector. However, I'm not satisfied with this solution and want to find a more elegant way.
One of my attempts was the following:
std::vector<byte> myVector;
&myVector[0] = (byte)"MyString";
which bugs and gets me an
error C2106: '=': left operand must be l-value
What is the correct way to do this?
The most basic thing would be something like:
const char *cstr = "bla"
std::vector<char> vec(cstr, cstr + strlen(cstr));
Of course, don't calculate the length if you know it.
The more common solution is to use the std::string class:
const char *cstr;
std::string str = cstr;
STL containers such as vector always take ownership, i.e. they manage their own memory. You cannot modify the memory managed internally by an STL container. For that reason, your attempt (and any similar attempt) is doomed to failure.
The only valid solution is to copy the memory or to write a new STL-style container that doesn’t take ownership of the memory it accesses.
Something along these lines should work
std::vector<byte> myVector = std::vector((byte*)cstring, (byte*)ctring + strlen(cstring))
Also, this is still going to just iterate through the c string and insert the values into the vector. Like Konrad said, that's just how you have to do it since vectors manage their own memory.
In case you would not like to copy the original string and would just like to iterate over it as an array of bytes, then C++20 has std::span to offer.
auto const const_bytes = std::as_bytes(std::span{str.data(), str.size()});
std::span<const std::byte> provides std::vector like iterating capabilities which I think is what you might be looking for.
Note: The original string would have to remain valid for the entire scope of std::span variable
std::vector<byte> myVector;
const char* str = "MyString";
myVector.assign( str, str+strlen(str) );
The most obvious question would be why you don't just use std::string:
std::string myString("MyString");
but if you really think you need a vector:
char myString[] = "MyString";
std::vector<byte> myVector(myString, myString+sizeof(myString));
You might also want to consider using std::tr1::array:
std::tr1::array<byte, sizeof("MyString")> myArray = {"MyString"};
C++ 0x will also have std::array.
const char *cstr = "bla"
std::vector<char> vec;
vec.resize(strlen(cstr)+1);
strcpy(&vec[0],cstr);
Const-correctness in C++ is still giving me headaches. In working with some old C code, I find myself needing to assign turn a C++ string object into a C string and assign it to a variable. However, the variable is a char * and c_str() returns a const char []. Is there a good way to get around this without having to roll my own function to do it?
edit: I am also trying to avoid calling new. I will gladly trade slightly more complicated code for less memory leaks.
C++17 and newer:
foo(s.data(), s.size());
C++11, C++14:
foo(&s[0], s.size());
However this needs a note of caution: The result of &s[0]/s.data()/s.c_str() is only guaranteed to be valid until any member function is invoked that might change the string. So you should not store the result of these operations anywhere. The safest is to be done with them at the end of the full expression, as my examples do.
Pre C++-11 answer:
Since for to me inexplicable reasons nobody answered this the way I do now, and since other questions are now being closed pointing to this one, I'll add this here, even though coming a year too late will mean that it hangs at the very bottom of the pile...
With C++03, std::string isn't guaranteed to store its characters in a contiguous piece of memory, and the result of c_str() doesn't need to point to the string's internal buffer, so the only way guaranteed to work is this:
std::vector<char> buffer(s.begin(), s.end());
foo(&buffer[0], buffer.size());
s.assign(buffer.begin(), buffer.end());
This is no longer true in C++11.
There is an important distinction you need to make here: is the char* to which you wish to assign this "morally constant"? That is, is casting away const-ness just a technicality, and you really will still treat the string as a const? In that case, you can use a cast - either C-style or a C++-style const_cast. As long as you (and anyone else who ever maintains this code) have the discipline to treat that char* as a const char*, you'll be fine, but the compiler will no longer be watching your back, so if you ever treat it as a non-const you may be modifying a buffer that something else in your code relies upon.
If your char* is going to be treated as non-const, and you intend to modify what it points to, you must copy the returned string, not cast away its const-ness.
I guess there is always strcpy.
Or use char* strings in the parts of your C++ code that must interface with the old stuff.
Or refactor the existing code to compile with the C++ compiler and then to use std:string.
There's always const_cast...
std::string s("hello world");
char *p = const_cast<char *>(s.c_str());
Of course, that's basically subverting the type system, but sometimes it's necessary when integrating with older code.
If you can afford extra allocation, instead of a recommended strcpy I would consider using std::vector<char> like this:
// suppose you have your string:
std::string some_string("hello world");
// you can make a vector from it like this:
std::vector<char> some_buffer(some_string.begin(), some_string.end());
// suppose your C function is declared like this:
// some_c_function(char *buffer);
// you can just pass this vector to it like this:
some_c_function(&some_buffer[0]);
// if that function wants a buffer size as well,
// just give it some_buffer.size()
To me this is a bit more of a C++ way than strcpy. Take a look at Meyers' Effective STL Item 16 for a much nicer explanation than I could ever provide.
You can use the copy method:
len = myStr.copy(cStr, myStr.length());
cStr[len] = '\0';
Where myStr is your C++ string and cStr a char * with at least myStr.length()+1 size. Also, len is of type size_t and is needed, because copy doesn't null-terminate cStr.
Just use const_cast<char*>(str.data())
Do not feel bad or weird about it, it's perfectly good style to do this.
It's guaranteed to work in C++11. The fact that it's const qualified at all is arguably a mistake by the original standard before it; in C++03 it was possible to implement string as a discontinuous list of memory, but no one ever did it. There is not a compiler on earth that implements string as anything other than a contiguous block of memory, so feel free to treat it as such with complete confidence.
If you know that the std::string is not going to change, a C-style cast will work.
std::string s("hello");
char *p = (char *)s.c_str();
Of course, p is pointing to some buffer managed by the std::string. If the std::string goes out of scope or the buffer is changed (i.e., written to), p will probably be invalid.
The safest thing to do would be to copy the string if refactoring the code is out of the question.
std::string vString;
vString.resize(256); // allocate some space, up to you
char* vStringPtr(&vString.front());
// assign the value to the string (by using a function that copies the value).
// don't exceed vString.size() here!
// now make sure you erase the extra capacity after the first encountered \0.
vString.erase(std::find(vString.begin(), vString.end(), 0), vString.end());
// and here you have the C++ string with the proper value and bounds.
This is how you turn a C++ string to a C string. But make sure you know what you're doing, as it's really easy to step out of bounds using raw string functions. There are moments when this is necessary.
If c_str() is returning to you a copy of the string object internal buffer, you can just use const_cast<>.
However, if c_str() is giving you direct access tot he string object internal buffer, make an explicit copy, instead of removing the const.
Since c_str() gives you direct const access to the data structure, you probably shouldn't cast it. The simplest way to do it without having to preallocate a buffer is to just use strdup.
char* tmpptr;
tmpptr = strdup(myStringVar.c_str();
oldfunction(tmpptr);
free tmpptr;
It's quick, easy, and correct.
In CPP, if you want a char * from a string.c_str()
(to give it for example to a function that only takes a char *),
you can cast it to char * directly to lose the const from .c_str()
Example:
launchGame((char *) string.c_str());
C++17 adds a char* string::data() noexcept overload. So if your string object isn't const, the pointer returned by data() isn't either and you can use that.
Is it really that difficult to do yourself?
#include <string>
#include <cstring>
char *convert(std::string str)
{
size_t len = str.length();
char *buf = new char[len + 1];
memcpy(buf, str.data(), len);
buf[len] = '\0';
return buf;
}
char *convert(std::string str, char *buf, size_t len)
{
memcpy(buf, str.data(), len - 1);
buf[len - 1] = '\0';
return buf;
}
// A crazy template solution to avoid passing in the array length
// but loses the ability to pass in a dynamically allocated buffer
template <size_t len>
char *convert(std::string str, char (&buf)[len])
{
memcpy(buf, str.data(), len - 1);
buf[len - 1] = '\0';
return buf;
}
Usage:
std::string str = "Hello";
// Use buffer we've allocated
char buf[10];
convert(str, buf);
// Use buffer allocated for us
char *buf = convert(str);
delete [] buf;
// Use dynamic buffer of known length
buf = new char[10];
convert(str, buf, 10);
delete [] buf;
I'm working in a C++ unmanaged project.
I need to know how can I take a string like this "some data to encrypt" and get a byte[] array which I'm gonna use as the source for Encrypt.
In C# I do
for (int i = 0; i < text.Length; i++)
buffer[i] = (byte)text[i];
What I need to know is how to do the same but using unmanaged C++.
Thanks!
If you just need read-only access, then c_str() will do it:
char const *c = myString.c_str();
If you need read/write access, then you can copy the string into a vector. vectors manage dynamic memory for you. You don't have to mess with allocation/deallocation then:
std::vector<char> bytes(myString.begin(), myString.end());
bytes.push_back('\0');
char *c = &bytes[0];
std::string::data would seem to be sufficient and most efficient. If you want to have non-const memory to manipulate (strange for encryption) you can copy the data to a buffer using memcpy:
unsigned char buffer[mystring.length()];
memcpy(buffer, mystring.data(), mystring.length());
STL fanboys would encourage you to use std::copy instead:
std::copy(mystring.begin(), mystring.end(), buffer);
but there really isn't much of an upside to this. If you need null termination use std::string::c_str() and the various string duplication techniques others have provided, but I'd generally avoid that and just query for the length. Particularly with cryptography you just know somebody is going to try to break it by shoving nulls in to it, and using std::string::data() discourages you from lazily making assumptions about the underlying bits in the string.
Normally, encryption functions take
encrypt(const void *ptr, size_t bufferSize);
as arguments. You can pass c_str and length directly:
encrypt(strng.c_str(), strng.length());
This way, extra space is allocated or wasted.
In C++17 and later you can use std::byte to represent actual byte data. I would recommend something like this:
std::vector<std::byte> to_bytes(std::string const& s)
{
std::vector<std::byte> bytes;
bytes.reserve(std::size(s));
std::transform(std::begin(s), std::end(s), std::back_inserter(bytes), [](char c){
return std::byte(c);
});
return bytes;
}
From a std::string you can use the c_ptr() method if you want to get at the char_t buffer pointer.
It looks like you just want copy the characters of the string into a new buffer. I would simply use the std::string::copy function:
length = str.copy( buffer, str.size() );
If you just need to read the data.
encrypt(str.data(),str.size());
If you need a read/write copy of the data put it into a vector. (Don;t dynamically allocate space that's the job of vector).
std::vector<byte> source(str.begin(),str.end());
encrypt(&source[0],source.size());
Of course we are all assuming that byte is a char!!!
If this is just plain vanilla C, then:
strcpy(buffer, text.c_str());
Assuming that buffer is allocated and large enough to hold the contents of 'text', which is the assumption in your original code.
If encrypt() takes a 'const char *' then you can use
encrypt(text.c_str())
and you do not need to copy the string.
You might go with range-based for loop, which would look like this:
std::vector<std::byte> getByteArray(const string& str)
{
std::vector<std::byte> buffer;
for (char str_char : str)
buffer.push_back(std::byte(str_char));
return buffer;
}
I dont think you want to use the c# code you have there. They provide System.Text.Encoding.ASCII(also UTF-*)
string str = "some text;
byte[] bytes = System.Text.Encoding.ASCII.GetBytes(str);
your problems stem from ignoring the encoding in c# not your c++ code