So recently I was playing with the concept of creating my own C++ classes that represent generic data (such as strings, numbers and arrays).
And so far my progress on this has been good (as seen here: https://github.com/LapysDev/LapysCPP).
Except one hitch. For the life of me, I can not figure out why the code below faults when it comes to creating a String class object with a variable amount of arguments.
#include <iostream>
#include <sstream>
#include <string.h>
// Make a new C-style string (or stringify a value).
char* stringify(char character) {
std::string stream = static_cast<std::ostringstream*>(&(std::ostringstream() << character)) -> str();
char* string = new char[stream.size() + 1];
strcpy(string, stream.c_str());
return string;
}
template <typename data> char* stringify(data string) { return strdup(std::string(string).c_str()); }
char* globalString = stringify("");
class String {
public:
char* value = stringify("");
String() {}
template <typename data>
String(data value) {
strcat(globalString, value);
this -> value = stringify(globalString);
globalString = stringify("");
}
template <typename data, typename... argumentsData>
String(data value, argumentsData... values) {
strcat(globalString, stringify(value));
String(values...);
}
};
int main(int argc, char* argv[]) {
std::cout << "String [1]: '" << String("Hello, World!").value << '\'' << std::endl;
// -> String [1]: 'Hello, World!'
std::cout << "String [3]: '" << String("Hello,", ' ', "World!").value << '\'';
// -> String [3]: ''
return 0;
}
I have tried everything I can with the code already (and yes, using an std::string for the text value is banned). If there's anyone out there that can explain why using multiple arguments faults when using char*'s, you're welcome to comment.
To summarize, I need to be able to create a String object with a proper value property using a variable amount of arguments.
// Works fine
String("Hello, World!").value // -> Hello, World!
// Needs fixing
String("Hello,", ' ', "World!").value // -> ...
I understand that this may not be platform to ask questions of this nature but a little help would go a long way. Thanks for reading through.
globalString is char* that points to the return value of stringify("");.
stringify("") returns strdup(std::string(string).c_str());. strdup returns a dynamically allocated string that has the length of its parameter ( and the same contents) .
Here, "" only contains \0 so the C-string returned from strdup will only have a length of 1.
You then try to call strcat(destination, source) with globalString as the destination, but globalString isn't big enough to fit the source.
strcat says:
The behavior is undefined if the destination array is not large
enough for the contents of both src and dest and the terminating null
character. The behavior is undefined if the strings overlap. The
behavior is undefined if either dest or src is not a pointer to a
null-terminated byte string.
So both of your test cases are UB. Even the first test that seems to work well.
std::string handles all of this for you. If you somehow aren't allowed to use it for whatever (stupid) reason a professor has given you, then make sure to allocate enough space for globalString before calling strcat on it, C-strings are tricky beasts.
Related
I have a const char pointer which I know for sure came from a string. For example:
std::string myString = "Hello World!";
const char* myCstring = myString.c_str();
In my case I know myCstring came from a string, but I no longer have access to that string (I received the const char* from a function call, and I cannot modify the function's argument list).
Given that I know myCstring points to contents of an existing string, is there any way to safely access the pointer of the parent string from which it originated? For example, could I do something like this?
std::string* hackyStringPointer = myCstring - 6; //Along with whatever pointer casting stuff may be needed
My concern is that perhaps the string's contents possibly cannot be guaranteed to be stored in contiguous memory on some or all platforms, etc.
Given that I know myCstring points to contents of an existing string, is there any way to safely access the pointer of the parent string from which it originated?
No, there is no way to obtain a valid std::string* pointer from a const char* pointer to character data that belongs to a std::string.
I received the const char* from a function call, and I cannot modify the function's argument list
Your only option in this situation would be if you can pass a pointer to the std::string itself as the actual const char* pointer, but that will only work if whatever is calling your function does not interpret the const char* in any way (and certainly not as a null-terminated C string), eg:
void doSomething(void (*func)(const char*), const char *data)
{
...
func(data);
...
}
void myFunc(const char *myCstring)
{
std::string* hackyStringPointer = reinterpret_cast<std::string*>(myCstring);
...
}
...
std::string myString = "Hello World!";
doSomething(&myFunc, reinterpret_cast<char*>(&myString));
You cannot convert a const char* that you get from std::string::c_str() to a std::string*. The reason you can't do this is because c_str() returns a pointer to the string data, not the string object itself.
If you are trying to get std::string so you can use it's member functions then what you can do is wrap myCstring in a std::string_view. This is a non-copying wrapper that lets you treat a c-string like it is a std::string. To do that you would need something like
std::string_view sv{myCstring, std::strlen(myCstring)};
// use sv here like it was a std::string
Yes (it seems), although I agree that if I need to do this it's likely a sign that my code needs reworking in general. Nevertheless, the answer seems to be that the string pointer resides 4 words before the const char* which c_str() returns, and I did recover a string* from a const char* belonging to a string.
#include <string>
#include <iostream>
std::string myString = "Hello World!";
const char* myCstring = myString.c_str();
unsigned int strPtrSize = sizeof(std::string*);
unsigned int cStrPtrSize = sizeof(const char*);
long strAddress = reinterpret_cast<std::size_t>(&myString);
long cStrAddress = reinterpret_cast<std::size_t>(myCstring);
long addressDifference = strAddress - cStrAddress;
long estStrAddress = cStrAddress + addressDifference;
std::string* hackyStringPointer = reinterpret_cast<std::string*>(estStrAddress);
cout << "Size of String* " << strPtrSize << ", Size of const char*: " << cStrPtrSize << "\n";
cout << "String Address: " << strAddress << ", C String Address: " << cStrAddress << "\n";
cout << "Address Difference: " << addressDifference << "\n";
cout << "Estimated String Address " << estStrAddress << "\n";
cout << "Hacky String: " << *hackyStringPointer << "\n";
//If any of these asserts trigger on any platform, I may need to re-evaluate my answer
assert(addressDifference == -4);
assert(strPtrSize == cStrPtrSize);
assert(hackyStringPointer == &myString);
The output of this is as follows:
Size of String* 4, Size of const char*: 4
String Address: 15725656, C String Address: 15725660
Address Difference: -4
Estimated String Address: 15725656
Hacky String: Hello World!
It seems to work so far. If someone can show that the address difference between a string and its c_str() can change over time on the same platform, or if all members of a string are not guaranteed to reside in contiguous memory, I'll change my answer to "No."
This reference says
The pointer returned may be invalidated by further calls to other member functions that modify the object.
You say you got the char* from a function call, this means you do not know what happens to the string in the mean time, is that right? If you know that the original string is not changed or deleted (e.g. gets out of scope and thus is destructed) then you can still use the char*.
Your example code however has multiple problems. You want to do this:
std::string* hackyStringPointer = myCstring - 6;
but I think you meant
char* hackyStringPointer = myCstring;
One, you cannot cast the char* to a string* and second you do not want to go BEFORE the start of the char*. The char* points to the first character of the string, you can use it to access the characters up to the trailing 0 character. But you should not go before the first or after the trailing 0 character though, as you do not know what is in that memory or if it even exists.
I have a function f() which receives a char* p and gives a const char* to it.
void f(char *p) {
string s = "def";
strcpy(p, s.c_str());
}
In the main() below I expect to get q = "def".
int main(){
char *q = "abc";
f(q);
cout << q << endl;
}
By running this I get segmentation fault and as I am new in C++ I don't understand why.
I also get a segmentation fault when I do not initialize q thus:
int main(){
char *q;
f(q);
cout << q << endl;
}
Knowing that the function's parameter and the way it's called must not change. Is there any work around that I can do inside the function? Any suggestions?
Thanks for your help.
You are trying to change a string literal. Any attemp to change a string literal results in undefined behaviour of the program.
Take into account that string literals have types of constant character arrays. So it would be more correct to write
const char *q = "abc";
From the C++ Standard (2.14.5 String literals)
8 Ordinary string literals and UTF-8 string literals are also referred
to as narrow string literals. A narrow string literal has type
“array of n const char”, where n is the size of the string as
defined below, and has static storage duration
You could write your program the following way
//...
void f(char *p) {
string s = "def";
strcpy(p, s.c_str());
}
//..
main(){
char q[] = "abc";
f(q);
cout << q << endl;
}
If you need to use a pointer then you could write
//...
void f(char *p) {
string s = "def";
strcpy(p, s.c_str());
}
//..
main(){
char *q = new char[4] { 'a', 'b', 'c', '\0' };
f(q);
cout << q << endl;
delete []q;
}
This is an issue that, in reality, should fail at compilation time but for really old legacy reasons they allow it.
"abc" is not not a mutable string and therefore it should be illegal to point a mutable pointer to it.
Really any legacy code that does this should be forced to be fixed, or have some pragma around it that lets it compile or some permissive flag set in the build.
But a long time ago in the old days of C there was no such thing as a const modifier, and literals were stored in char * parameters and programmers had to be careful what they did with them.
The latter construct, where q is not initialised at all is an error because now q could be pointing anywhere, and is unlikely to be pointing to a valid memory place to write the string. It is actually undefined behaviour, for obvious reason - who knows where q is pointing?
The normal construct for such a function like f is to request not only a pointer to a writable buffer but also a maximum available size (capacity). Usually this size includes the null-terminator, sometimes it might not, but either way the function f can then write into it without an issue. It will also often return a status, possibly the number of bytes it wanted to write. This is very common for a "C" interface. (And C interfaces are often used even in C++ for better portability / compatibility with other languages).
To make it work in this instance, you need at least 4 bytes in your buffer.
int main()
{
char q[4];
f(q);
std::cout << q << std::endl;
}
would work.
Inside the function f you can use std::string::copy to copy into the buffer. Let's modify f.
(We assume this is a prototype and in reality you have a meaningful name and it returns something more meaningful that you retrieve off somewhere).
size_t f( char * buf, size_t capacity )
{
std::string s = "def";
size_t copied = s.copy( buf, capacity-1 ); // leave a space for the null
buf[copied] = '\0'; // std::string::copy doesn't write this
return s.size() + 1; // the number of bytes you need
}
int main()
{
char q[3];
size_t needed = f( q, 3 );
std::cout << q << " - needed " << needed << " bytes " << std::endl;
}
Output should be:
de needed 4 bytes
In your question you suggested you can change your function but not the way it is called. Well in that case, you actually have only one real solution:
void f( const char * & p )
{
p = "def";
}
Now you can happily do
int main()
{
const char * q;
f( q );
std::cout << q << std::endl;
}
Note that in this solution I am actually moving your pointer to point to something else. This works because it is a static string. You cannot have a local std::string then point it to its c_str(). You can have a std::string whose lifetime remains beyond the scope of your q pointer e.g. stored in a collection somewhere.
Look at the warnings you get while compiling your code (and if you don’t get any, turn up the warning levels or get a better compiler).
You will notice that despite the type declaration, the value of q is not really mutable. The compiler was humoring you because not doing so would break a lot of legacy code.
You can't do that because you assigned a string literal to your char*. And this is memory you can't modify.
With your f, You should do
int main(){
char q[4 /*or more*/];
f(q);
std::cout << q << std::endl;
}
The problem is that you are trying to write on a read-only place in the process address space. As all the string literals are placed in read-only-data. char *q = "abc"; creates a pointer and points towards the read-only section where the string literal is placed. and when you copy using strcpy or memcpy or even try q[1] = 'x' it attempts to write on a space which is write protected.
This was the problem among many other solutions one can be
main(){
char *q = "abc"; \\ the strings are placed at a read-only portion
\\ in the process address space so you can not write that
q = new char[4]; \\ will make q point at space at heap which is writable
f(q);
cout << q << endl;
delete[] q;
}
the initialization of q is unnecessary here. after the second line q gets a space of 4 characters on the heap (3 for chars and one for null char). This would work but there are many other and better solutions to this problem which varies from situation to situation.
I'm a C/C++ beginner trying to build what seems like a pretty simple program: it loads a file into a c-string (const char*). However, although the program is incredibly simple, it's not working in a way I understand. Take a look:
#include <iostream>
#include <fstream>
std::string loadStringFromFile(const char* file)
{
std::ifstream shader_file(file, std::ifstream::in);
std::string str((std::istreambuf_iterator<char>(shader_file)), std::istreambuf_iterator<char>());
return str;
}
const char* loadCStringFromFile(const char* file)
{
std::ifstream shader_file(file, std::ifstream::in);
std::string str((std::istreambuf_iterator<char>(shader_file)), std::istreambuf_iterator<char>());
return str.c_str();
}
int main()
{
std::string hello = loadStringFromFile("hello.txt");
std::cout << "hello: " << hello.c_str() << std::endl;
const char* hello2 = loadCStringFromFile("hello.txt");
std::cout << "hello2: " << hello2 << std::endl;
hello2 = hello.c_str();
std::cout << "hello2 = hello.c_str(), hello2: " << hello2 << std::endl;
return 0;
}
The output looks like this:
hello: Heeeeyyyyyy
hello2: 青!
hello2 = hello, hello2: Heeeeyyyyyy
The initial hello2 value changes every time, always some random kanji (I'm using a Japanese computer, so I'm guessing that's why it's kanji).
In my naive view, it seems like the two values should print identically. One function returns a c++ string, which I then convert to a c-string, and the other loads the string, converts the c-string from that and returns it. I made sure that the string was loading properly in loadCStringFromFile by couting the value before I returned it, and indeed it was what I had thought, e.g.:
/*(inside loadCStringFromFile)*/
const char* result = str.c_str();
std::cout << result << std::endl;//prints out "Heeeyyyyyy" as expected
return result;
So why should the value change? Thanks for the help...
Your problem is that str in loadCStringFromFile is a local variable, and is destructed when the function returns. At that point the return value from c_str() is invalid.
More detail here
Your first function, loadStringFromFile, is a more C++-like way of doing it, and illustrates the benefit of having a class manage memory for you. If you use char* then you have to take much more care where memory is allocated and freed.
the function
std::string loadStringFromFile(const char* file)
returns a string copy of the string created inside the function which is copied before the string goes out of scope i.e. the function ends, that is why it works.
const char* loadCStringFromFile(const char* file)
on the other hand returns a pointer to the local string which goes out of scope when the function returns and is destroyed so the returned address, the const char*, points to somewhere undefined.
in order for the second way to work you either need to create the string before calling the function :
const char* loadCStringFromFile(const char* file, string& str); // return str.c_str()
..
string str;
const char* result = loadCStringFromFile(file,str);
or you create a string on the heap in the function and pass the address back, but that gets a bit messy since the caller would need to delete the string to avoid memleak.
You should duplicate output of str.c_str():
return strdup(str.c_str);
function strdup can be found in cstring header.
Is there an exact equivalent to strncpy in the C++ Standard Library? I mean a function, that copies a string from one buffer to another until it hits the terminating 0? For instance when I have to parse strings from an unsafe source, such as TCP packets, so I'm able to perform checks in length while coping the data.
I already searched a lot regarding this topic and I also found some interesting topics, but all of those people were happy with std::string::assign, which is also able to take a size of characters to copy as a parameter. My problem with this function is, that it doesn't perform any checks if a terminating null was already hit - it takes the given size serious and copies the data just like memcpy would do it into the string's buffer. This way there is much more memory allocated and copied than it had to be done, if there were such a check while coping.
That's the way I'm working around this problem currently, but there is some overhead I'd wish to avoid:
// Get RVA of export name
const ExportDirectory_t *pED = (const ExportDirectory_t*)rva2ptr(exportRVA);
sSRA nameSra = rva2sra(pED->Name);
// Copy it into my buffer
char *szExportName = new char[nameSra.numBytesToSectionsEnd];
strncpy(szExportName,
nameSra.pSection->pRawData->constPtr<char>(nameSra.offset),
nameSra.numBytesToSectionsEnd);
szExportName[nameSra.numBytesToSectionsEnd - 1] = 0;
m_exportName = szExportName;
delete [] szExportName;
This piece of code is part of my parser for PE-binaries (of the routine parsing the export table, to be exact). rva2sra converts a relative virtual address into a PE-section relative address. The ExportDirectory_t structure contains the RVA to the export name of the binary, which should be a zero-terminated string. But that doesn't always have to be the case - if someone would like it, it would be able to omit the terminating zero which would make my program run into memory which doesn't belong to the section, where it would finally crash (in the best case...).
It wouldn't be a big problem to implement such a function by myself, but I'd prefer it if there were a solution for this implemented in the C++ Standard Library.
If you know that the buffer you want to make a string out of has at least one NUL in it then you can just pass it to the constructor:
const char[] buffer = "hello\0there";
std::string s(buffer);
// s contains "hello"
If you're not sure, then you just have to search the string for the first null, and tell the constructor of string to make a copy of that much data:
int len_of_buffer = something;
const char* buffer = somethingelse;
const char* copyupto = std::find(buffer, buffer + len_of_buffer, 0); // find the first NUL
std::string s(buffer, copyupto);
// s now contains all the characters up to the first NUL from buffer, or if there
// was no NUL, it contains the entire contents of buffer
You can wrap the second version (which always works, even if there isn't a NUL in the buffer) up into a tidy little function:
std::string string_ncopy(const char* buffer, std::size_t buffer_size) {
const char* copyupto = std::find(buffer, buffer + buffer_size, 0);
return std::string(buffer, copyupto);
}
But one thing to note: if you hand the single-argument constructor a const char* by itself, it will go until it finds a NUL. It is important that you know there is at least one NUL in the buffer if you use the single-argument constructor of std::string.
Unfortunately (or fortunately), there is no built in perfect equivalent of strncpy for std::string.
The std::string class in STL can contain null characters within the string ("xxx\0yyy" is a perfectly valid string of length 7). This means that it doesn't know anything about null termination (well almost, there are conversions from/to C strings). In other words, there's no alternative in the STL for strncpy.
There are a few ways to still accomplish your goal with a shorter code:
const char *ptr = nameSra.pSection->pRawData->constPtr<char>(nameSra.offset);
m_exportName.assign(ptr, strnlen(ptr, nameSra.numBytesToSectionsEnd));
or
const char *ptr = nameSra.pSection->pRawData->constPtr<char>(nameSra.offset);
m_exportName.reserve(nameSra.numBytesToSectionsEnd);
for (int i = 0; i < nameSra.numBytesToSectionsEnd && ptr[i]; i++)
m_exportName += ptr[i];
Is there an exact equivalent to strncpy in the C++ Standard Library?
I certainly hope not!
I mean a function, that copies a string from one buffer to another until it hits the terminating 0?
Ah, but that's not what strncpy() does -- or at least it's not all it does.
strncpy() lets you specify the size, n, of the destination buffer, and copies at most n characters. That's fine as far as it goes. If the length of the source string ("length" defined as the number of characters preceding the terminating '\0') exceeds n, the destination buffer is padded with additional \0's, something that's rarely useful. And if the length if the source string exceeds n, then the terminating '\0' is not copied.
The strncpy() function was designed for the way early Unix systems stored file names in directory entries: as a 14-byte fixed-size buffer that can hold up to a 14-character name. (EDIT: I'm not 100% sure that was the actual motivation for its design.) It's arguably not a string function, and it's not just a "safer" variant of strcpy().
You can achieve the equivalent of what one might assume strncpy() does (given the name) using strncat():
char dest[SOME_SIZE];
dest[0] = '\0';
strncat(dest, source_string, SOME_SIZE);
This will always '\0'-terminate the destination buffer, and it won't needlessly pad it with extra '\0' bytes.
Are you really looking for a std::string equivalent of that?
EDIT : After I wrote the above, I posted this rant on my blog.
There is no built-in equivalent. You have to roll your own strncpy.
#include <cstring>
#include <string>
std::string strncpy(const char* str, const size_t n)
{
if (str == NULL || n == 0)
{
return std::string();
}
return std::string(str, std::min(std::strlen(str), n));
}
The string's substring constructor can do what you want, although it's not an exact equivalent of strncpy (see my notes at the end):
std::string( const std::string& other,
size_type pos,
size_type count = std::string::npos,
const Allocator& alloc = Allocator() );
Constructs the string with a substring [pos, pos+count) of other. If count == npos or if the requested substring lasts past the end of the string, the resulting substring is [pos, size()).
Source: http://www.cplusplus.com/reference/string/string/string/
Example:
#include <iostream>
#include <string>
#include <cstring>
int main ()
{
std::string s0 ("Initial string");
std::string s1 (s0, 0, 40); // count is bigger than s0's length
std::string s2 (40, 'a'); // the 'a' characters will be overwritten
strncpy(&s2[0], s0.c_str(), s2.size());
std::cout << "s1: '" << s1 << "' (size=" << s1.size() << ")" << std::endl;
std::cout << "s2: '" << s2 << "' (size=" << s2.size() << ")" << std::endl;
return 0;
}
Output:
s1: 'Initial string' (size=14)
s2: 'Initial string' (size=40)
Differences with strncpy:
the string constructor always appends a null-terminating character to the result, strncpy does not;
the string constructor does not pad the result with 0s if a null-terminating character is reached before the requested count, strncpy does.
Use the class' constructor:
string::string str1("Hello world!");
string::string str2(str1);
This will yield an exact copy, as per this documentation: http://www.cplusplus.com/reference/string/string/string/
std::string has a constructor with next signature that can be used :
string ( const char * s, size_t n );
with next description:
Content is initialized to a copy of the string formed by the first n characters in the array of characters pointed by s.
I have char* which is of fixed (known) width but is not null terminated.
I want to pass it into LOG4CPLUS_ERROR("Bad string " << char_pointer); but as its not null terminated it will print it all.
Any suggestions of some light weight way of getting "(char[length])*char_pointer" without performing a copy?
No, you'll have to deep-copy and null-terminate it. That code expects a null-terminated string and it means a contiguous block of characters ending with a null terminator.
If your goal is to print such a string, you could:
Store the last byte.
Replace it with \0.
Print the string.
Print the stored byte.
Put the stored byte back into the last position of the string.
Wrap all this in a function.
Real iostreams
When you're writing to a real iostream, then you can just use ostream.write() which takes a char* and a size for how many bytes to write -- no null termination necessary. (In fact, any null characters in the string would be written to the ostream, and would not be used to determine the size.)
Logging libraries
In some logging libraries, the stream that you write to is not a real iostream. This is the case in Log4CPP.
However, in Log4CPlus which is what it appears matt is using, the object that you're writing to is a std::basic_ostringstream<tchar> (see loggingmacros.h and streams.h for the definition, since none of this is obvious from the documentation). There's just one problem: in the macro LOG4CPLUS_ERROR, the first << is already built into the macro, so he won't be able to call LOG4CPLUS_ERROR(.write(char_pointer,length)) or anything like that. Unfortunately, I don't see an easy way around this without deconstructing the LOG4CPLUS_ERROR error macro and getting into the internals of Log4CPlus
Solution
I'm not sure why you're trying to avoid copying the string at this point, since you can see that there's a lot of copying going on inside the logging library. Any attempt to avoid that extra string copy is probably unwarranted optimization.
I'm going to assume that it's an issue of code cleanliness, and maybe an issue of making sure the copy happens inside the LOG4CPLUS_ERROR macro, as opposed to outside it. In that case, just use:
LOG4CPLUS_ERROR("Bad string " << std::string(char_pointer, length));
We're getting hung up on the semantics of conversion between char* and char[]. Take a step back, what are you trying to do? If this is a simple case of on an error condition, streaming out the content of a structure to a stream, why not do it properly?
e.g.
struct foo
{
char a1[10];
char a2[10];
char a3[10];
char a4[10];
};
// free function to stream the above structure properly..
std::ostream operator<<(std::ostream& str, foo const& st)
{
str << "foo::a1[";
std::copy(st.a1, st.a1 + sizeof(st.a1), std::ostream_iterator<char>(str));
str << "]\n";
str << "foo::a2[";
std::copy(st.a2, st.a2 + sizeof(st.a2), std::ostream_iterator<char>(str));
str << "]\n";
:
return str;
}
Now you can simply stream out an instance of foo and don't have to worry about null terminated string etc.!
I keep a string reference class in my toolkit just for these type of situations. Here is a greatly abbreviated version of that class. I trimmed away anything that is not relevant to this particular problem:
#include <iostream>
class stringref {
public:
stringref(const char* ptr, unsigned len) : ptr(ptr), len(len) {}
unsigned length() { return len; }
const char* data() { return ptr; }
private:
const char* ptr;
unsigned len;
};
std::ostream& operator<< (std::ostream& os, stringref sr) {
const char* data = sr.data();
for (unsigned len = sr.length(); len--; )
os << *data++;
return os;
}
using namespace std;
int main (int argc, const char * argv[])
{
cout << "string: " << stringref("test", 4) << endl;
}
or, in your case:
LOG4CPLUS_ERROR("Bad string " << stringref(char_pointer, length));
should work.
The idea of a string reference class is to keep enough information about a string (a size and a pointer) to refer to any block of memory which represents a string. It relies on you to make sure that the underlying string data is valid throughout the lifetime of a stringref object. This way you can pass around and process string information with a minimum of overhead.
When you know it is of fixed length: Why not simply add one more charakter to the size of the array? Then you can easily fill this last char with \0 terminating character and all will be fine
No, you'll have to copy it. There is no proper conversion in the language that you can use to get the array type out of it.
It seems very odd that you want to do this, or that you have a non-terminated C-style string in the first place.
Why are you not using std::string?