char* pStr = new String("Hello");
char* s = "Hello";
Is the first one correct? Are there any difference between these two? My guess is that the first one is allocated on the heap,and the other one an the stack.Am i correct or are there any other differences?
The first one is just incorrect and won't compile because there is no such thing as String in either C or C++. The second one will compile, and is fine in C(afaik). In C++, however, the conversion from a string literal to char* is deprecated. You can unintentionally write later s[0] = 'X'; which is undefined behavior.
The correct way of doing it is using const (in C++)
const char * s = "Hello";
or, better, use string
std::string s("Hello");
pStr and s are pointers, so it is important to distinguish between the pointers themselves and the data that they point to.
On the first line, pStr is a pointer to an instance of the String class allocated on the heap. The string data inside this instance is a copy of a literal string "Hello" that is stored in the program's data segment. The copying is done by the String constructor. (You've referred to a String class, but I assume you mean std::string).
On the second line, s is a pointer to data stored in the program's data segment. Data in the data segment is immutable, so s should really be const char *.
There isn't enough information in your example to tell whether pStr and s are stored on the heap or the stack. If they are variables inside a function then they are on the stack. If there are members of a class then they are on the heap if the class was instantiated on the help (using new) or on the stack if it is instantiated as a value.
The line
char* pStr = new std::string("Hello");
will cause a compiler semantic error, because the LHS has a type of char* and the RHS has a type of std::string.
The line
char* s = "Hello"
will compile, but may give a warning, because the LHS has a type of char* and the RHS has a type of const char*.
Related
Why does the following code in C work?
const char* str = NULL;
str = "test";
str = "test2";
Since str is a pointer to a constant character, why are we allowed to assign it different string literals? Further, how can we protect str from being modified? It seems like this could be a problem if, for example, we later assigned str to a longer string which ended up writing over another portion of memory.
I should add that in my test, I printed out the memory address of str before and after each of my assignments and it never changed. So, although str is a pointer to a const char, the memory is actually being modified. I wondered if perhaps this is a legacy issue with C?
You are changing the pointer, which is not const (the thing it's pointing to is const).
If you want the pointer itself to be const, the declaration would look like:
char * const str = "something";
or
char const * const str = "something"; // a const pointer to const char
const char * const str = "something"; // same thing
Const pointers to non-const data are usually a less useful construct than pointer-to-const.
Further, how can we protect str from being modified?
char * const str1; // str1 cannot be modified, but the character pointed to can
const char * str2; // str2 can be modified, but the character pointed to cannot
const char * const str3 // neither str3 nor the character pointed to can be modified.
The easiest way to read this is to start from the variable name and read to the left:
str1 is a constant pointer to a character
str2 is a pointer to a character constant
str3 is a constant pointer to a character constant
NOTE: the right-to-left reading does not work in the general case, but for simple declarations it's a simple way to do it. I found a java applet based on code from "The C Programming Language" that can decipher declarations with a full explanation of how to do it.
On a related note, definitely take a look at "const pointer versus pointer to const". It helps with what some people call const correctness. I keep it in my bookmarks so that I can refer to it every now and then.
What you're looking for may be the syntax...
const char* const str = NULL;
str = "test";
str = "test2";
Notice the "const" after the char* which yields a compiler error when trying to compile/build.
Memory for the string literals are allocated on the stack, and all your assignments do are change the str pointer to point to those memory addresses. The constant character it pointed to initially hasn't changed at all.
Besides, declaring a variable as const means that variable is read-only; it does not mean the value is constant!
string str;
char *a=str.c_str();
This code is working fine for me but every place else I see this code instead
string str;
char *a=new char[str.length()];
strcpy(a,str.c_str());
I wonder which one is correct and why?
Assuming that the type of str is std::string, neither of the code is are correct.
char *a=str.c_str();
is invalid because c_str() will return const char* and removing const without casting (usually const_cast) is invalid.
char *a=new char[str.length()];
strcpy(a,str.c_str());
is invalid because str.length() don't count the terminating null-character while allocating for terminating null-character is required to use strcpy().
There are no dangling pointer problem in code posted here because no pointers are invalidated here.
The two code segments do different things.
The first assigns the pointer value of str to your new c-tpye string, and implicitly converts from const char*(c_str() return type) to char*, which is wrong. If you were to change your new string you would face an error. Even if c_str() returned char*, altering the new string would also make changes in str.
The second on the other hand creates a new c-type string from the original string, copying it byte-by-byte to the new memory allocated for your new string.
Although the line of code you wrote is incorrect, as it does not cover the terminating null character of a c-type string \0. In order to fix that, allocate 1 extra byte for it:
char *a=new char[str.length()+1];
After copying the data from the first string to your new one, making alterations to it will not result in changes in the original str.
Possibly.
Consider this.
char const* get_string() {
string str{"Hello"};
return str.c_str();
}
That function returns a pointer to the internal value of str, which goes out of scope when the function returns. You have a dangling pointer. Undefined behaviour. Watch out for time-travelling nasal monkeys.
Now consider this.
char const* get_string() {
string str{"Hello"};
char const* a = new char[str.length()+1];
strcpy(a, str.c_str());
return a;
}
That function returns a valid pointer to a valid null-terminated C-style string. No dangling pointer. If you forget to delete[] it you will have a memory leak, but that's not what you asked about.
The difference is one of object lifetime. Be aware of scope.
So this article is discussing the use of declaring a string literal like const char* foo = "foo" it ends with the claim:
const char *foo = "foo";
is almost never what you want. Instead, you want to use one of the following forms:
For a string meant to be exported:
const char foo[] = "foo";
For a string meant to be used in the same source file:
static const char foo[] = "foo";
For a string meant to be used across several source files for the same library:
__attribute__((visibility("hidden"))) const char foo[] = "foo";
My understanding here is that const char* const foo = "foo" is equivalent to const char foo[] = "foo" simply because we're talking about a C-string pointer that can never be changed to point at anything else, whereas const char* foo = "foo" could be used to point at any other C-String.
Is this an accurate synopsis? Always use either const char* const or const char[]?
Let's get pedantic here.
char const * const p_foo = "foo";
The above defines a {constant} pointer to the {constant} character literal "foo". The pointer is to the single first character of the character literal.
const char bar[] = "bar";
The above defines a character array.
The character array is *read-only".
The character array is the length of the text literal "bar" plus a
nul terminator (4 characters).
The contents of the text literal are copied into the array. (The
compiler may optimize this step away).
Fundamentally, you have the difference between a pointer to the first character of a literal and an array.
The pointer is pointing to a single character. Incrementing the pointer may not point to a valid entity (since it is not an array, but a pointer to a single datum). There is an underlying assumption that the pointer can be incremented to the next character.
With an array you know that there are more than one character sequentially in memory (provided the array is of length 2 or more). You don't know if there is a terminating nul in the sequence (collection). You can assume that, but an array of characters does not guarantee that.
Usages
With the array declaration, the length of the text is known at compile time.
With the pointer declaration, you would need to use strlen to determine the length of the text at run-time. The run-time code doesn't know the length of the target data string; only a length of 1 can be guaranteed.
Sometimes, using static and const can help the compiler optimize.
For example:
static const char moo[] = "moo";
allows the compiler to access the text directly without creating an array variable and copying the text into the variable.
In a function that receives a pointer to a character, you can't guarantee that the pointer points to a valid location (the content of the pointer can be invalid).
Each declaration has its benefits and side-effects.
The choice is yours.
As Thomas Matthews' answer states both a const char* and a const char* const are pointers, and a const char[] is an array.
But as justified here there are 3 problems with using pointers:
Memory for the pointer's storage is required
The indirection incurred by the pointer is required
A pointer requires separate storage of an end pointer for the array or an array size
Ultimately as justified in the link:
The simple answer is that when declaring a variable you should prefer a const char[].
I do agree that an array decays into a pointer when being evaluated but there are a few functionalities that come only with an array. For example when you declare an array you have additional information as to what the size of the array is.
Also, for the fixed array case, memory is allocated specifically for foo. So you are allowed to change the contents of the array like you usually can and the array is destroyed, deallocating memory when it runs out of scope (typical local variable).
When you define it as a pointer, the compiler places foo into read-only memory, and then points to it (usually). Note that this is why most cases constant strings are defined as char* and even the compiler would warn you when you set it as a non constant pointer.
#include <iostream>
int main()
{
char* a = "foo";
return 0;
}
This code would throw you a warning like:
ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
char* a = "foo";
and any change you try to make to the string would typically lead to a segmentation fault.
in c++ is this a good practice to initialize char array with string?
such as:
char* abc = (char *) ("abcabc");
I see a lot of these in my co-worker's code. Should I change it to the right practice?
such as
std::string abc_str = "abcabc";
const char* abc= abc_str .c_str();
This statement
char* abc = (char *) ("abcabc");
is simply bad. String literals in C++ have types of constant character arrays. So a valid declaration will look like
const char *abc = "abcabc";
Note: In C you indeed may write
char *abc = "abcabc";
Nevertheless string literals are immutable. Any attempt to modify a string literal results in undefined behaviour.
By the way there is no any character array that is initialized by a string literal.:) Maybe you mean the following
char abc[] = "abcabc";
Using standard class std::string does not exclude using character arrays and moreover pointers to string literals.
Take into account that these declarations
const char *abc = "abcabc";
and
std::string abc_str = "abcabc";
const char* abc= abc_str .c_str();
are not equivalent. Relative to the first declaration string literals have static storage duration and their addresses are not changed during the program execution.
In the second declaration pointer abc points to dynamically allocated memory that can be reallocated if object abc_str will be changed. In this case the pointer will be invalid.
Also the first declaration supposes that the array (string literal) pointed to by the pointer will not be changed. In the second declaration it is supposed that the object of type std::string will be changed. Otherwise there is no great sense to declare an object of type std::string instead of the pointer.
Thus the meanings of the declarations are simply different.
char* abc = (char *) ("abcabc");
That is bad. Don't do it.
You are treating a string literal that is not supposed to be modified like it can be modified.
After that,
abc[0] = 'd';
will be OK by the compiler but not OK at run time. What you need to use is:
char abc[] = "abcabc";
This will create an array that is modifiable.
Both of those are bad.
char* abc = (char*) ("abcabc");
A string literal is a constant and, as such, may be stored in write protected memory. Therefore writing to it can crash your program, it is undefined behaviour.
Rather than cast away the constness you should keep it const and make a copy if you want to edit its contents.
const char* abc = "abcabc";
The other one should be avoided too:
std::string abc_str = "abcabc";
const char* abc = abc_str.c_str();
Keeping it const is good but if the string is changed it could be reallocated to another place in memory leaving your pointer dangling.
Also in pre C++11 code the pointer stops being valid the second it is assigned because there is no guarantee it is not a temporary.
Better to call abc_str.c_str() each time.
The chances are that because c_str() is such a trivial operation it will be optimized away by the compiler making it just as efficient as using the raw pointer.
Instead of both of those what you should be doing is using std::string all the way. If you absolutely need a const char* (for old legacy code) you can obtain it using c_str().
std::string abc_str = "abcabc"; // this is perfect why do more?
old_horrible_function(abc_str.c_str()); // only when needed
Here I've two lines of code
const char * s1 = "test";
char s2 [] = "test";
Both lines of code have the same behavior, so I cannot see any difference whether I should prefer s1 over s2 or vice-versa. In addition to s1 and s2, there is also the way of using std::string. I think the way of using std::string is the most elegant. While looking at other code, I often see that people either use const char * or char s []. Thus, my question is now, when should I use const char * s1 or char s [] or std::string? What are the differences and in which situations should I use which approach?
POINTERS
--------
char const* s1 = "test"; // pointer to string literal - do not modify!
char* s1 = "test"; // pointer to string literal - do not modify!
// (conversion to non-const deprecated in C++03 and
// disallowed in C++11)
ARRAYS
------
char s1[5] = "test"; // mutable character array copied from string literal
// - do what you like with it!
char s1[] = "test"; // as above, but with size deduced from initialisation
CLASS-TYPE OBJECTS
------------------
std::string s1 = "test"; // C++ string object with data copied from string
// literal - almost always what you *really* want
const char * s1 = "test";
char s2 [] = "test";
These two aren't identical. s1 is immutable: it points to constant memory. Modifying string literals is undefined behaviour.
And yes, in C++ you should prefer std::string.
The first one is constant, the second isn't. std::string is a class type and implements many useful functions and methods for string manipulation, making it much easier and user-friendly. The c-style 'strings' with char pointers are difficult to control, manipulate and often cause errors, but don't have the overhead the std::string has. Generally it's better to stick to the std::strings cause they're easier to maintain.
The only difference between the two that you should care about is this:
Which one is your project already using?
These two do not have the same behavior. s1 is a simple pointer which is initialized to point to some (usually read-only) area of the memory. s2, on the other hand, defines a local array of size 5, and fills it with a copy of this string.
Formally, you are not allowed to modify s1, that is, do something like s1[0] = 'a'. In particular, under weird circumstances, it could cause all other "test"s in your program to become "aest", because they all share the same memory. This is the reason modern compilers yell when you write
char* s = "test";
On the other hand, modifying s2 is allowed, since it is a local copy.
In other words, in the following example,
const char* s1 = "test";
const char* s2 = "test";
char s3[] = "test";
char s4[] = "test";
s1 and s2 may very well point to the same address in memory, while s3 and s4 are two different copies of the same string, and reside in different areas of memory.
If you're writing C++, use std::string unless you absolutely need an array of characters. If you need a modifiable array of characters, use char s[]. If you only need an immutable string, use const char*.
Use std::string unless you know why you need a char array / pointer to char.
which one to be used depends upon your requirement. Pointer offers you more flexiblity. and in some cases vulerability. Strings are a safe option and they provide Iterator support.