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If I declare a function:
vector<int> & call(string *target)
How should I use target in the function for a comparison with another string? For example:
string str;
if(str == //string in target){
...
}
As &target, or simply target? Also, how should the return look? I'm assuming it should be:
return &some_vector;
since that is the type in the function declaration. Finally, what about the opposite? That is:
vector<int> & call(string &target)
When in the function and wanting to use the string, is it as simple as:
*target
On your first question it's
if(str == *target){
On your second question my advice would simply be, don't. It looks like you are trying to return a reference to a local variable. That is a well known newbie mistake that will simply crash your program.
On your third question it's
if(str == target){
You need to dereference the pointer to get the object. That is, use *:
if (str == *target) {
Also, how should the return look?
You're returning a reference, not a pointer. That means you simply need to return the name of the object:
return some_vector;
Finally, what about the opposite?
If target is a reference (NOT a pointer) than you simply use the name of the object. There's no dereference involved with this.
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I'm trying to make a login/register project and I have difficulties in declaring the char* tempUsername from this code (SIGSEVG segmentation fault)
char *tempUsername, *tempPassword, *tempPasswordConfirm, *tempSecurityQuestion;
/*
no other declaration for tempUsername here
*/
std::cout<<"Enter your new username:\n";
std::cin>>tempUsername;
//process stops here
if(fileSearch(newFilename(tempUsername))) {
std::cout<<"Username already exists! Choose another username!\n";
}
else {
std::cout<<"Enter your password:\n";
std::cin>>tempPassword;
std::cout<<"Confirm your password:\n";
I'm having a hard time understanding anything about pointers, so any advice is more than helpful!
char *tempUsername
std::cin>>tempUsername;
The problem here is that your pointer is uninitialised. When you try to extract from the input stream into the uninitialised pointer, the behaviour of the program will be undefined. Don't do this.
Your goal seems to be to read a string of user input. A solution that I can recommend is to use the std::string class:
std::string tempUsername;
std::cin >> tempUsername;
No need to use a pointer to achieve this.
I can use a char* as an array of chars, is that true?
It is not true in general. If you have a char* that points to an element of an array of char, then you can use that char* as an iterator to access the elements of the array.
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I am trying initialize element of array
array[m][n] == char("X");
after printing that element i'm getting the value of it equals д (Russian d); how deal with it, and I'm not even able to initialize that element without parsing const char to char.
You have to write simply as
array[m][n] = 'X';
where 'X' is a character literal.
Or if you like very much string literals then:)
array[m][n] = *"X";
or
array[m][n] = "X"[0];
EDIT: I am sorry. You have also to use the assignmnet operator (=) instead of the comparison operator (==)
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I am new to C++ and I need to create a function with this structure:
BOOL myfunc(LPDWORD myLpdword){ // myLpdword must be an out parameter
DWORD myDword = 1;
myLpdword = &myDword;
return true;
}
int main(){
DWORD outDword = 20;
myfunc(&outDword);
cout << outDword << end1;
return 0;
}
I expected that the value of outDword would be 1 (changed by myfunc), but the value is not changed by myfunc.
Please, can you give me a hint to solve this problem?
Like this
BOOL myfunc(LPDWORD myLpdword){ // myLpdword must be an out parameter
*myLpdword = 1;
return true;
}
Out parameter is not something that means anything in C++. MS use it but only because they are using a consistent terminology across different languages.
In C++ terms what you did is pass a pointer to the variable you want to modify to myfunc. What the above code does is take that pointer and dereference with the * operator it to modify the variable you wanted modified.
I like that you're writing small test programs to check your understanding of C++. But as others said there's no real substitute for a decent book. Any C++ book is going to cover this.
You passed in a pointer to outDword.
myLpdword is now a pointer to outDword.
You then changed myLpdword to be a pointer to myDword.
You never did anything with the VALUE of outDword.
You assigned the pointer of a variable that will not exist after exiting the function body (read on scopes in C/C++.
To solve your problem, assign the value of the variable to the dereferenced pointer, like so: *myLpdword = myDword;. It would also be wise to check that the value of the pointer is not null before dereferencing it like this: if (myLpdword == 0) { return; } . This check doesn't guarantee that the pointer is safe to assign to, but Atleast guards you against null pointer access.
In C++ this is called pass-by-reference. You denote it with an ampersand in the function signature:
bool myfunc(DWORD &myDword) {…
The ampersands you are using now are actually getting the address of the variables, so you should remove those.
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Using the following code:
class MyClass
{
public:
void GetStrings(list<std::string>& strs)
{
strs = m_strings;
}
list<std::string> m_strings;
}
Are all m_strings's elements being copied when strs = m_strings; is performed?
In other words:
Is this equal to list<std::string>& strs = m_strings; or to list<std::string> strs = m_strings; (with respect to elements copy)
yes.
Now, if you wanted to copy just a reference to the list (like you have in languages such as C# or Java) then pass a shared_ptr instead, then you will have a single list with 2 references to it.
All variable assignment in C/C++ is done by value, even if the variable being passed is a pointer (eg a 4 byte variable). Other languages are the same, its just that they cover this up with their language constructs.
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For example:
string str[3];
void foo(char** str)
{
//do something to str...
}
How to pass str[] to function foo in a convenient way?
The function expects an array of pointers, so you'll have to make one from your array of strings:
std::vector<char*> pointers;
for (auto & s : str) {
pointers.push_back(&s[0]);
}
foo(&pointers[0]);
Beware that this may not be valid if the function modifies the pointers, or the strings they point to. A better option would be to avoid mixing C and C++ style string handling, if possible.