If you just want to pass a std::string to a function that needs const char* you can use
std::string str;
const char * c = str.c_str();
If you want to get a writable copy, like char *, you can do that with this:
std::string str;
char * writable = new char[str.size() + 1];
std::copy(str.begin(), str.end(), writable);
writable[str.size()] = '\0'; // don't forget the terminating 0
// don't forget to free the string after finished using it
delete[] writable;
Edit: Notice that the above is not exception safe. If anything between the new call and the delete call throws, you will leak memory, as nothing will call delete for you automatically. There are two immediate ways to solve this.
boost::scoped_array will delete the memory for you upon going out of scope:
std::string str;
boost::scoped_array<char> writable(new char[str.size() + 1]);
std::copy(str.begin(), str.end(), writable.get());
writable[str.size()] = '\0'; // don't forget the terminating 0
// get the char* using writable.get()
// memory is automatically freed if the smart pointer goes
// out of scope
This is the standard way (does not require any external library). You use std::vector, which completely manages the memory for you.
std::string str;
std::vector<char> writable(str.begin(), str.end());
writable.push_back('\0');
// get the char* using &writable[0] or &*writable.begin()
Given say...
std::string x = "hello";
How to get a character pointer that's valid while x remains in scope and isn't modified further
C++11 simplifies things; the following all give access to the same internal string buffer:
const char* p_c_str = x.c_str();
const char* p_data = x.data();
char* p_writable_data = x.data(); // for non-const x from C++17
const char* p_x0 = &x[0];
char* p_x0_rw = &x[0]; // compiles iff x is not const...
All the above pointers will hold the same value - the address of the first character in the buffer. Even an empty string has a "first character in the buffer", because C++11 guarantees to always keep an extra NUL/0 terminator character after the explicitly assigned string content (e.g. std::string("this\0that", 9) will have a buffer holding "this\0that\0").
Given any of the above pointers:
char c = p[n]; // valid for n <= x.size()
// i.e. you can safely read the NUL at p[x.size()]
Only for the non-const pointer p_writable_data and from &x[0]:
p_writable_data[n] = c;
p_x0_rw[n] = c; // valid for n <= x.size() - 1
// i.e. don't overwrite the implementation maintained NUL
Writing a NUL elsewhere in the string does not change the string's size(); string's are allowed to contain any number of NULs - they are given no special treatment by std::string (same in C++03).
In C++03, things were considerably more complicated (key differences highlighted):
x.data()
const char* to the string's internal buffer which wasn't required by the Standard to conclude with a NUL (i.e. might be ['h', 'e', 'l', 'l', 'o'] followed by uninitialised or garbage values, with accidental accesses thereto having undefined behaviour).
x.size() characters are safe to read, i.e. x[0] through x[x.size() - 1]&x[0]
f(const char* p, size_t n) { if (n == 0) return; ...whatever... } you mustn't call f(&x[0], x.size()); when x.empty() - just use f(x.data(), ...).x.data() but:
const x this yields a non-const char* pointer; you can overwrite string contentx.c_str()
const char* to an ASCIIZ (NUL-terminated) representation of the value (i.e. ['h', 'e', 'l', 'l', 'o', '\0']).x.data() and &x[0]x.size() + 1 characters are safe to read.Whichever way you get a pointer, you must not access memory further along from the pointer than the characters guaranteed present in the descriptions above. Attempts to do so have undefined behaviour, with a very real chance of application crashes and garbage results even for reads, and additionally wholesale data, stack corruption and/or security vulnerabilities for writes.
If you call some string member function that modifies the string or reserves further capacity, any pointer values returned beforehand by any of the above methods are invalidated. You can use those methods again to get another pointer. (The rules are the same as for iterators into strings).
See also How to get a character pointer valid even after x leaves scope or is modified further below....
From C++11, use .c_str() for ASCIIZ data, and .data() for "binary" data (explained further below).
In C++03, use .c_str() unless certain that .data() is adequate, and prefer .data() over &x[0] as it's safe for empty strings....
...try to understand the program enough to use data() when appropriate, or you'll probably make other mistakes...
The ASCII NUL '\0' character guaranteed by .c_str() is used by many functions as a sentinel value denoting the end of relevant and safe-to-access data. This applies to both C++-only functions like say fstream::fstream(const char* filename, ...) and shared-with-C functions like strchr(), and printf().
Given C++03's .c_str()'s guarantees about the returned buffer are a super-set of .data()'s, you can always safely use .c_str(), but people sometimes don't because:
.data() communicates to other programmers reading the source code that the data is not ASCIIZ (rather, you're using the string to store a block of data (which sometimes isn't even really textual)), or that you're passing it to another function that treats it as a block of "binary" data. This can be a crucial insight in ensuring that other programmers' code changes continue to handle the data properly.string implementation will need to do some extra memory allocation and/or data copying in order to prepare the NUL terminated bufferAs a further hint, if a function's parameters require the (const) char* but don't insist on getting x.size(), the function probably needs an ASCIIZ input, so .c_str() is a good choice (the function needs to know where the text terminates somehow, so if it's not a separate parameter it can only be a convention like a length-prefix or sentinel or some fixed expected length).
x leaves scope or is modified furtherYou'll need to copy the contents of the string x to a new memory area outside x. This external buffer could be in many places such as another string or character array variable, it may or may not have a different lifetime than x due to being in a different scope (e.g. namespace, global, static, heap, shared memory, memory mapped file).
To copy the text from std::string x into an independent character array:
// USING ANOTHER STRING - AUTO MEMORY MANAGEMENT, EXCEPTION SAFE
std::string old_x = x;
// - old_x will not be affected by subsequent modifications to x...
// - you can use `&old_x[0]` to get a writable char* to old_x's textual content
// - you can use resize() to reduce/expand the string
// - resizing isn't possible from within a function passed only the char* address
std::string old_x = x.c_str(); // old_x will terminate early if x embeds NUL
// Copies ASCIIZ data but could be less efficient as it needs to scan memory to
// find the NUL terminator indicating string length before allocating that amount
// of memory to copy into, or more efficient if it ends up allocating/copying a
// lot less content.
// Example, x == "ab\0cd" -> old_x == "ab".
// USING A VECTOR OF CHAR - AUTO, EXCEPTION SAFE, HINTS AT BINARY CONTENT, GUARANTEED CONTIGUOUS EVEN IN C++03
std::vector<char> old_x(x.data(), x.data() + x.size()); // without the NUL
std::vector<char> old_x(x.c_str(), x.c_str() + x.size() + 1); // with the NUL
// USING STACK WHERE MAXIMUM SIZE OF x IS KNOWN TO BE COMPILE-TIME CONSTANT "N"
// (a bit dangerous, as "known" things are sometimes wrong and often become wrong)
char y[N + 1];
strcpy(y, x.c_str());
// USING STACK WHERE UNEXPECTEDLY LONG x IS TRUNCATED (e.g. Hello\0->Hel\0)
char y[N + 1];
strncpy(y, x.c_str(), N); // copy at most N, zero-padding if shorter
y[N] = '\0'; // ensure NUL terminated
// USING THE STACK TO HANDLE x OF UNKNOWN (BUT SANE) LENGTH
char* y = alloca(x.size() + 1);
strcpy(y, x.c_str());
// USING THE STACK TO HANDLE x OF UNKNOWN LENGTH (NON-STANDARD GCC EXTENSION)
char y[x.size() + 1];
strcpy(y, x.c_str());
// USING new/delete HEAP MEMORY, MANUAL DEALLOC, NO INHERENT EXCEPTION SAFETY
char* y = new char[x.size() + 1];
strcpy(y, x.c_str());
// or as a one-liner: char* y = strcpy(new char[x.size() + 1], x.c_str());
// use y...
delete[] y; // make sure no break, return, throw or branching bypasses this
// USING new/delete HEAP MEMORY, SMART POINTER DEALLOCATION, EXCEPTION SAFE
// see boost shared_array usage in Johannes Schaub's answer
// USING malloc/free HEAP MEMORY, MANUAL DEALLOC, NO INHERENT EXCEPTION SAFETY
char* y = strdup(x.c_str());
// use y...
free(y);
char* or const char* generated from a stringSo, above you've seen how to get a (const) char*, and how to make a copy of the text independent of the original string, but what can you do with it? A random smattering of examples...
string's text, as in printf("x is '%s'", x.c_str());x's text to a buffer specified by your function's caller (e.g. strncpy(callers_buffer, callers_buffer_size, x.c_str())), or volatile memory used for device I/O (e.g. for (const char* p = x.c_str(); *p; ++p) *p_device = *p;)x's text to an character array already containing some ASCIIZ text (e.g. strcat(other_buffer, x.c_str())) - be careful not to overrun the buffer (in many situations you may need to use strncat)const char* or char* from a function (perhaps for historical reasons - client's using your existing API - or for C compatibility you don't want to return a std::string, but do want to copy your string's data somewhere for the caller)
string variable to which that pointer pointed has left scopestd::string implementations (e.g. STLport and compiler-native) may pass data as ASCIIZ to avoid conflictsGiven say...
std::string x = "hello";
How to get a character pointer that's valid while x remains in scope and isn't modified further
C++11 simplifies things; the following all give access to the same internal string buffer:
const char* p_c_str = x.c_str();
const char* p_data = x.data();
char* p_writable_data = x.data(); // for non-const x from C++17
const char* p_x0 = &x[0];
char* p_x0_rw = &x[0]; // compiles iff x is not const...
All the above pointers will hold the same value - the address of the first character in the buffer. Even an empty string has a "first character in the buffer", because C++11 guarantees to always keep an extra NUL/0 terminator character after the explicitly assigned string content (e.g. std::string("this\0that", 9) will have a buffer holding "this\0that\0").
Given any of the above pointers:
char c = p[n]; // valid for n <= x.size()
// i.e. you can safely read the NUL at p[x.size()]
Only for the non-const pointer p_writable_data and from &x[0]:
p_writable_data[n] = c;
p_x0_rw[n] = c; // valid for n <= x.size() - 1
// i.e. don't overwrite the implementation maintained NUL
Writing a NUL elsewhere in the string does not change the string's size(); string's are allowed to contain any number of NULs - they are given no special treatment by std::string (same in C++03).
In C++03, things were considerably more complicated (key differences highlighted):
x.data()
const char* to the string's internal buffer which wasn't required by the Standard to conclude with a NUL (i.e. might be ['h', 'e', 'l', 'l', 'o'] followed by uninitialised or garbage values, with accidental accesses thereto having undefined behaviour).
x.size() characters are safe to read, i.e. x[0] through x[x.size() - 1]&x[0]
f(const char* p, size_t n) { if (n == 0) return; ...whatever... } you mustn't call f(&x[0], x.size()); when x.empty() - just use f(x.data(), ...).x.data() but:
const x this yields a non-const char* pointer; you can overwrite string contentx.c_str()
const char* to an ASCIIZ (NUL-terminated) representation of the value (i.e. ['h', 'e', 'l', 'l', 'o', '\0']).x.data() and &x[0]x.size() + 1 characters are safe to read.Whichever way you get a pointer, you must not access memory further along from the pointer than the characters guaranteed present in the descriptions above. Attempts to do so have undefined behaviour, with a very real chance of application crashes and garbage results even for reads, and additionally wholesale data, stack corruption and/or security vulnerabilities for writes.
If you call some string member function that modifies the string or reserves further capacity, any pointer values returned beforehand by any of the above methods are invalidated. You can use those methods again to get another pointer. (The rules are the same as for iterators into strings).
See also How to get a character pointer valid even after x leaves scope or is modified further below....
From C++11, use .c_str() for ASCIIZ data, and .data() for "binary" data (explained further below).
In C++03, use .c_str() unless certain that .data() is adequate, and prefer .data() over &x[0] as it's safe for empty strings....
...try to understand the program enough to use data() when appropriate, or you'll probably make other mistakes...
The ASCII NUL '\0' character guaranteed by .c_str() is used by many functions as a sentinel value denoting the end of relevant and safe-to-access data. This applies to both C++-only functions like say fstream::fstream(const char* filename, ...) and shared-with-C functions like strchr(), and printf().
Given C++03's .c_str()'s guarantees about the returned buffer are a super-set of .data()'s, you can always safely use .c_str(), but people sometimes don't because:
.data() communicates to other programmers reading the source code that the data is not ASCIIZ (rather, you're using the string to store a block of data (which sometimes isn't even really textual)), or that you're passing it to another function that treats it as a block of "binary" data. This can be a crucial insight in ensuring that other programmers' code changes continue to handle the data properly.string implementation will need to do some extra memory allocation and/or data copying in order to prepare the NUL terminated bufferAs a further hint, if a function's parameters require the (const) char* but don't insist on getting x.size(), the function probably needs an ASCIIZ input, so .c_str() is a good choice (the function needs to know where the text terminates somehow, so if it's not a separate parameter it can only be a convention like a length-prefix or sentinel or some fixed expected length).
x leaves scope or is modified furtherYou'll need to copy the contents of the string x to a new memory area outside x. This external buffer could be in many places such as another string or character array variable, it may or may not have a different lifetime than x due to being in a different scope (e.g. namespace, global, static, heap, shared memory, memory mapped file).
To copy the text from std::string x into an independent character array:
// USING ANOTHER STRING - AUTO MEMORY MANAGEMENT, EXCEPTION SAFE
std::string old_x = x;
// - old_x will not be affected by subsequent modifications to x...
// - you can use `&old_x[0]` to get a writable char* to old_x's textual content
// - you can use resize() to reduce/expand the string
// - resizing isn't possible from within a function passed only the char* address
std::string old_x = x.c_str(); // old_x will terminate early if x embeds NUL
// Copies ASCIIZ data but could be less efficient as it needs to scan memory to
// find the NUL terminator indicating string length before allocating that amount
// of memory to copy into, or more efficient if it ends up allocating/copying a
// lot less content.
// Example, x == "ab\0cd" -> old_x == "ab".
// USING A VECTOR OF CHAR - AUTO, EXCEPTION SAFE, HINTS AT BINARY CONTENT, GUARANTEED CONTIGUOUS EVEN IN C++03
std::vector<char> old_x(x.data(), x.data() + x.size()); // without the NUL
std::vector<char> old_x(x.c_str(), x.c_str() + x.size() + 1); // with the NUL
// USING STACK WHERE MAXIMUM SIZE OF x IS KNOWN TO BE COMPILE-TIME CONSTANT "N"
// (a bit dangerous, as "known" things are sometimes wrong and often become wrong)
char y[N + 1];
strcpy(y, x.c_str());
// USING STACK WHERE UNEXPECTEDLY LONG x IS TRUNCATED (e.g. Hello\0->Hel\0)
char y[N + 1];
strncpy(y, x.c_str(), N); // copy at most N, zero-padding if shorter
y[N] = '\0'; // ensure NUL terminated
// USING THE STACK TO HANDLE x OF UNKNOWN (BUT SANE) LENGTH
char* y = alloca(x.size() + 1);
strcpy(y, x.c_str());
// USING THE STACK TO HANDLE x OF UNKNOWN LENGTH (NON-STANDARD GCC EXTENSION)
char y[x.size() + 1];
strcpy(y, x.c_str());
// USING new/delete HEAP MEMORY, MANUAL DEALLOC, NO INHERENT EXCEPTION SAFETY
char* y = new char[x.size() + 1];
strcpy(y, x.c_str());
// or as a one-liner: char* y = strcpy(new char[x.size() + 1], x.c_str());
// use y...
delete[] y; // make sure no break, return, throw or branching bypasses this
// USING new/delete HEAP MEMORY, SMART POINTER DEALLOCATION, EXCEPTION SAFE
// see boost shared_array usage in Johannes Schaub's answer
// USING malloc/free HEAP MEMORY, MANUAL DEALLOC, NO INHERENT EXCEPTION SAFETY
char* y = strdup(x.c_str());
// use y...
free(y);
char* or const char* generated from a stringSo, above you've seen how to get a (const) char*, and how to make a copy of the text independent of the original string, but what can you do with it? A random smattering of examples...
string's text, as in printf("x is '%s'", x.c_str());x's text to a buffer specified by your function's caller (e.g. strncpy(callers_buffer, callers_buffer_size, x.c_str())), or volatile memory used for device I/O (e.g. for (const char* p = x.c_str(); *p; ++p) *p_device = *p;)x's text to an character array already containing some ASCIIZ text (e.g. strcat(other_buffer, x.c_str())) - be careful not to overrun the buffer (in many situations you may need to use strncat)const char* or char* from a function (perhaps for historical reasons - client's using your existing API - or for C compatibility you don't want to return a std::string, but do want to copy your string's data somewhere for the caller)
string variable to which that pointer pointed has left scopestd::string implementations (e.g. STLport and compiler-native) may pass data as ASCIIZ to avoid conflictsTry this
std::string s(reinterpret_cast<const char *>(Data), Size);
C++17 (upcoming standard) changes the synopsis of the template basic_string adding a non const overload of data():
charT* data() noexcept;Returns: A pointer p such that p + i == &operator for each i in [0,size()].
CharT const * from std::basic_string<CharT>std::string const cstr = { "..." };
char const * p = cstr.data(); // or .c_str()
CharT * from std::basic_string<CharT>std::string str = { "..." };
char * p = str.data();
CharT const * from std::basic_string<CharT>std::string str = { "..." };
str.c_str();
CharT * from std::basic_string<CharT>From C++11 onwards, the standard says:
- The char-like objects in a
basic_stringobject shall be stored contiguously. That is, for anybasic_stringobjects, the identity&*(s.begin() + n) == &*s.begin() + nshall hold for all values ofnsuch that0 <= n < s.size().
const_reference operator[](size_type pos) const;
reference operator[](size_type pos);Returns:
*(begin() + pos)ifpos < size(), otherwise a reference to an object of typeCharTwith valueCharT(); the referenced value shall not be modified.
const charT* c_str() const noexcept;const charT* data() const noexcept;Returns: A pointer p such that
p + i == &operator[](i)for eachiin[0,size()].
There are severable possible ways to get a non const character pointer.
std::string foo{"text"};
auto p = &*foo.begin();
Pro
Cons
'\0' is not to be altered / not necessarily part of the non-const memory.std::vector<CharT>std::string foo{"text"};
std::vector<char> fcv(foo.data(), foo.data()+foo.size()+1u);
auto p = fcv.data();
Pro
Cons
std::array<CharT, N> if N is compile time constant (and small enough)std::string foo{"text"};
std::array<char, 5u> fca;
std::copy(foo.data(), foo.data()+foo.size()+1u, fca.begin());
Pro
Cons
std::string foo{ "text" };
auto p = std::make_unique<char[]>(foo.size()+1u);
std::copy(foo.data(), foo.data() + foo.size() + 1u, &p[0]);
Pro
Cons
std::string foo{ "text" };
char * p = nullptr;
try
{
p = new char[foo.size() + 1u];
std::copy(foo.data(), foo.data() + foo.size() + 1u, p);
// handle stuff with p
delete[] p;
}
catch (...)
{
if (p) { delete[] p; }
throw;
}
Pro
Con
char* result = strcpy((char*)malloc(str.length()+1), str.c_str());
Use the .c_str() method for const char *.
You can use &mystring[0] to get a char * pointer, but there are a couple of gotcha's: you won't necessarily get a zero terminated string, and you won't be able to change the string's size. You especially have to be careful not to add characters past the end of the string or you'll get a buffer overrun (and probable crash).
There was no guarantee that all of the characters would be part of the same contiguous buffer until C++11, but in practice all known implementations of std::string worked that way anyway; see Does “&s[0]” point to contiguous characters in a std::string?.
Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved. Best to use them immediately and then discard.
Use the .c_str() method for const char *.
You can use &mystring[0] to get a char * pointer, but there are a couple of gotcha's: you won't necessarily get a zero terminated string, and you won't be able to change the string's size. You especially have to be careful not to add characters past the end of the string or you'll get a buffer overrun (and probable crash).
There was no guarantee that all of the characters would be part of the same contiguous buffer until C++11, but in practice all known implementations of std::string worked that way anyway; see Does “&s[0]” point to contiguous characters in a std::string?.
Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved. Best to use them immediately and then discard.
Just see this:
string str1("stackoverflow");
const char * str2 = str1.c_str();
However, note that this will return a const char *.
For a char *, use strcpy to copy it into another char array.
Use the .c_str() method for const char *.
You can use &mystring[0] to get a char * pointer, but there are a couple of gotcha's: you won't necessarily get a zero terminated string, and you won't be able to change the string's size. You especially have to be careful not to add characters past the end of the string or you'll get a buffer overrun (and probable crash).
There was no guarantee that all of the characters would be part of the same contiguous buffer until C++11, but in practice all known implementations of std::string worked that way anyway; see Does “&s[0]” point to contiguous characters in a std::string?.
Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved. Best to use them immediately and then discard.
I am working with an API with a lot of functions get as an input a char*.
I have created a small class to face this kind of problem, I have implemented the RAII idiom.
class DeepString
{
DeepString(const DeepString& other);
DeepString& operator=(const DeepString& other);
char* internal_;
public:
explicit DeepString( const string& toCopy):
internal_(new char[toCopy.size()+1])
{
strcpy(internal_,toCopy.c_str());
}
~DeepString() { delete[] internal_; }
char* str() const { return internal_; }
const char* c_str() const { return internal_; }
};
And you can use it as:
void aFunctionAPI(char* input);
// other stuff
aFunctionAPI("Foo"); //this call is not safe. if the function modified the
//literal string the program will crash
std::string myFoo("Foo");
aFunctionAPI(myFoo.c_str()); //this is not compiling
aFunctionAPI(const_cast<char*>(myFoo.c_str())); //this is not safe std::string
//implement reference counting and
//it may change the value of other
//strings as well.
DeepString myDeepFoo(myFoo);
aFunctionAPI(myFoo.str()); //this is fine
I have called the class DeepString because it is creating a deep and unique copy (the DeepString is not copyable) of an existing string.
C++17 (upcoming standard) changes the synopsis of the template basic_string adding a non const overload of data():
charT* data() noexcept;Returns: A pointer p such that p + i == &operator for each i in [0,size()].
CharT const * from std::basic_string<CharT>std::string const cstr = { "..." };
char const * p = cstr.data(); // or .c_str()
CharT * from std::basic_string<CharT>std::string str = { "..." };
char * p = str.data();
CharT const * from std::basic_string<CharT>std::string str = { "..." };
str.c_str();
CharT * from std::basic_string<CharT>From C++11 onwards, the standard says:
- The char-like objects in a
basic_stringobject shall be stored contiguously. That is, for anybasic_stringobjects, the identity&*(s.begin() + n) == &*s.begin() + nshall hold for all values ofnsuch that0 <= n < s.size().
const_reference operator[](size_type pos) const;
reference operator[](size_type pos);Returns:
*(begin() + pos)ifpos < size(), otherwise a reference to an object of typeCharTwith valueCharT(); the referenced value shall not be modified.
const charT* c_str() const noexcept;const charT* data() const noexcept;Returns: A pointer p such that
p + i == &operator[](i)for eachiin[0,size()].
There are severable possible ways to get a non const character pointer.
std::string foo{"text"};
auto p = &*foo.begin();
Pro
Cons
'\0' is not to be altered / not necessarily part of the non-const memory.std::vector<CharT>std::string foo{"text"};
std::vector<char> fcv(foo.data(), foo.data()+foo.size()+1u);
auto p = fcv.data();
Pro
Cons
std::array<CharT, N> if N is compile time constant (and small enough)std::string foo{"text"};
std::array<char, 5u> fca;
std::copy(foo.data(), foo.data()+foo.size()+1u, fca.begin());
Pro
Cons
std::string foo{ "text" };
auto p = std::make_unique<char[]>(foo.size()+1u);
std::copy(foo.data(), foo.data() + foo.size() + 1u, &p[0]);
Pro
Cons
std::string foo{ "text" };
char * p = nullptr;
try
{
p = new char[foo.size() + 1u];
std::copy(foo.data(), foo.data() + foo.size() + 1u, p);
// handle stuff with p
delete[] p;
}
catch (...)
{
if (p) { delete[] p; }
throw;
}
Pro
Con
I am working with an API with a lot of functions get as an input a char*.
I have created a small class to face this kind of problem, I have implemented the RAII idiom.
class DeepString
{
DeepString(const DeepString& other);
DeepString& operator=(const DeepString& other);
char* internal_;
public:
explicit DeepString( const string& toCopy):
internal_(new char[toCopy.size()+1])
{
strcpy(internal_,toCopy.c_str());
}
~DeepString() { delete[] internal_; }
char* str() const { return internal_; }
const char* c_str() const { return internal_; }
};
And you can use it as:
void aFunctionAPI(char* input);
// other stuff
aFunctionAPI("Foo"); //this call is not safe. if the function modified the
//literal string the program will crash
std::string myFoo("Foo");
aFunctionAPI(myFoo.c_str()); //this is not compiling
aFunctionAPI(const_cast<char*>(myFoo.c_str())); //this is not safe std::string
//implement reference counting and
//it may change the value of other
//strings as well.
DeepString myDeepFoo(myFoo);
aFunctionAPI(myFoo.str()); //this is fine
I have called the class DeepString because it is creating a deep and unique copy (the DeepString is not copyable) of an existing string.
Use the .c_str() method for const char *.
You can use &mystring[0] to get a char * pointer, but there are a couple of gotcha's: you won't necessarily get a zero terminated string, and you won't be able to change the string's size. You especially have to be careful not to add characters past the end of the string or you'll get a buffer overrun (and probable crash).
There was no guarantee that all of the characters would be part of the same contiguous buffer until C++11, but in practice all known implementations of std::string worked that way anyway; see Does “&s[0]” point to contiguous characters in a std::string?.
Note that many string member functions will reallocate the internal buffer and invalidate any pointers you might have saved. Best to use them immediately and then discard.
Just see this:
string str1("stackoverflow");
const char * str2 = str1.c_str();
However, note that this will return a const char *.
For a char *, use strcpy to copy it into another char array.
Try this
std::string s(reinterpret_cast<const char *>(Data), Size);
Source: Stackoverflow.com