I have the following in my WinRT component:
public value struct WinRTStruct
{
int x;
int y;
};
public ref class WinRTComponent sealed
{
public:
WinRTComponent();
int TestPointerParam(WinRTStruct * wintRTStruct);
};
int WinRTComponent::TestPointerParam(WinRTStruct * wintRTStruct)
{
wintRTStruct->y = wintRTStruct->y + 100;
return wintRTStruct->x;
}
But, it seems that the value of winRTStruct->y and x are always 0 inside the method, when called from C#:
WinRTComponent comp = new WinRTComponent();
WinRTStruct winRTStruct;
winRTStruct.x = 100;
winRTStruct.y = 200;
comp.TestPointerParam(out winRTStruct);
textBlock8.Text = winRTStruct.y.ToString();
What is the correct way to pass a struct by reference so it an be updated inside the method of a WinRTComponent written in C++/CX?
You cannot pass a struct by reference. All value types (including structs) in winrt are passed by value. Winrt structs are expected to be relatively small - they're intended to be used for holding things like Point and Rect.
In your case, you've indicated that the struct is an "out" parameter - an "out" parameter is write-only, its contents are ignored on input and are copied out on return. If you want a structure to be in and out, split it into two parameters - one "in" parameter and another "out" parameter (in/out parameters are not allowed in WinRT because they don't project to JS the way you expect them to project).
My co-worker helped me solve this.
In WinRT components, it seems that the best way to do this is to define a ref struct instead of a value struct:
public ref struct WinRTStruct2 sealed
{
private: int _x;
public:
property int X
{
int get(){ return _x; }
void set(int value){ _x = value; }
}
private: int _y;
public:
property int Y
{
int get(){ return _y; }
void set(int value){ _y = value; }
}
};
But this creates other problems. Now the VS11 compiler gives INTERNAL COMPILER ERROR when I try to add a method to the ref struct that returns an instance of the struct.
Related
I have a class that contains a lambda function that looks like this:
class Foo {
private:
inline static std::string text{};
public:
template<typename...T>
inline static auto func = [](T&&...args) mutable throw() {
text += std::string(args...);
};
void show() {
std::cout << text << '\n';
}
};
My intended use would be something like this:
int main() {
Foo bar;
bar.func<std::string, int, std::string>( "Hello, I am", 39, "years old!");
bar.show();
return 0;
}
I want the templated variadic lambda to take in any type of parameter that is a basic type such as string, char*, char[], int, float, double, etc... and to convert all of them into a single std::string that will be stored within the class.
When I run my code as such:
int main() {
Foo bar;
bar.func<string>( "Hello world!");
bar.show();
return 0;
}
Everything compiles fine, however, when I begin to add in various types such as the example from the intended use above, it fails to compile. Microsoft Visual Studio is giving me a C2400 compiler error: cannot convert from initialize list to std::string. No constructor could take the source type, or constructor overload resolution was ambiguous...
I believe I understand why it is ambiguous as that's not so much the issue. My question is what would be the proper and efficient way of using "move semantics or perfect forwarding"? I'm trying to avoid a bunch of copies of temporaries.
You could use fold expressions:
template<typename...T>
inline static auto func = [](T&&...args) mutable throw() {
text += (toString(args) + ...);
};
where toString is defined as:
template<class T>
std::string toString(T&& t){
if constexpr (std::is_arithmetic_v<std::decay_t<T>>)
return std::to_string(std::forward<T>(t));
else
return std::forward<T>(t);
}
you can extend toString to handle all types you need to convert to string.
Demo
i am new in this and i am working on App of media player and recording app. in which i have shown song list of device in the listview and recording start / stop / play. Now i want to convert that .mp3 recorded file into .mp4 and one image will show on behalf of a video in that file. Help me to achive this i have no idea and i refer many links and i didnt find anything.
Please check this link for your first question:
Why can't we initialize class members at their declaration?
Usually constructor is use to initialize value to data variables of class.
For 2nd Question:
If data member is not initialize after creation of object, It will contain garbage value. So initialize or assign suitable value to as per your need.
Check below code:
#include<iostream>
using namespace std;
class swap_values
{
int a, b, temp;
public:
swap_values(){
a=0;b=0;temp=0;
}
swap_values(int x, int y){
a = x;
b = y;
temp = 0;
}
void swapped()
{
temp = b;
b=a;
a=temp;
}
void print(){
cout<<"a: "<<a<<" b: "<<b<<endl;
}
};
int main()
{
int x =10; int y = 20;
swap_values obj(x, y);
obj.print();
obj.swapped();
obj.print();
return 0;
}
Everything can be done in better ways but just using your code this will work for you -
#include <iostream>
using namespace std;
class Swap {
private:
int a,b,temp;
public:
Swap()
{
a=10;
b=20;
temp=0;
}
void swapNums()
{
temp=a; a=b; b=temp;
cout<<a<<" " <<b<<endl;
}
};
int main() {
Swap s;
s.swapNums();
return 0;
}
You can avoid using class name as some function name. You can instead use constructor without a return type where you can initialise the member variables. swap method looks fine.
i am not able to initialize my variable in class.
class swap
{
int a=10; \\cannot declare here
int b=20; \\ cannot declare here
}
Since C++11, this is fine, you can have default member initialization.
The error is due to missing semicolon after }.
why it has garbage value with b ??
a=b;
b=temp;
temp=a;
Since temp was never initialized before assigning it to b, temp has an indeterminate value.
Any usage will lead to undefined behavior.
Here's a simple Swap struct:
struct Swap
{
int a = 10; // default member initialization
int b = 20; // default member initialization
Swap(int a = 20, int b = 10): a(b), b(a) {}; // swap on initialization
// using member initializer list
};
Swap s;
std::cout << s.a // 20
<< s.b // 10
<< std::endl;
In this example, default member initialization is "obsolete" / "redundant" due to member initializer list.
When one create a new thread, using ThreadStart() how does one pass multiple arguments to the function?
Here's an example:
using namespace System;
using namespace System::Threading;
public ref class Animal
{
public:
void Hungry(Object^ food, int quantity);
};
void Animal::Hungry(Object^ food, int quantity)
{
Console::WriteLine("The animal eats " + quantity.ToString() + food);
}
void main()
{
Animal^ test = gcnew Animal;
Thread^ threads = gcnew Thread(gcnew ParameterizedThreadStart(test, &Animal::Hungry));
threads->Start("Grass", 1); //need to pass 2nd argument!
}
It works fine with just one argument (if I remove int quantity and just have Object^ food) since ParameterizedThreadStart takes only an Object^
Like in any other situation where you have to put multiple values into one object, you either:
create a wrapper class or struct (the clean way)
or use some predefined one like Tuple (the lazy way)
Here's the lazy way:
void Animal::Hungry(Object^ param)
{
auto args = safe_cast<Tuple<String^, int>^>(param);
Console::WriteLine("The animal eats {1} {0}", args->Item1, args->Item2);
}
void main()
{
Animal^ test = gcnew Animal;
Thread^ threads = gcnew Thread(gcnew ParameterizedThreadStart(test, &Animal::Hungry));
threads->Start(Tuple::Create("Grass", 1));
}
I have a question concerning working with classes in c++. I must say I'm a beginner. For example, i have this class:
class student {
private:
char* name;
public:
int nrcrt;
student() {
name = new char[7];
name = "Anonim";
nrcrt = 0;
}
student(char* n, int n) {
this->name = new char[7];
strcpy(name, n);
nrcrt = nr;
}
~student() {
delete [] name;
}
char* get_name() {
return this->name;
}
}
void main() {
student group[3];
group[0] = student("Ana", 1);
group[1] = student("Alex", 2);
group[2] = student("Liam", 5);
for (i=0; i<3; i++) {
if (group.nrcrt[i] != 0)
cout << group[i].get_name() << Endl;
}
}
My question is why is it displaying different characters?
first of all your code is not working.
3.cpp:40:18: error: request for member ‘nrcrt’ in ‘group’, which is of non-class type ‘student [3]’
if(group.nrcrt[i]!=0)
i is also not declared.please make proper changes.
group.nrcrt[i]
should be changed to:
group[i].nrcrt
When the array is created, your default constructor is used.
When you assign to the elements, your destructor is called, deleting name.
The default constructor is assigning a literal to name, and deleting that memory has undefined behaviour.
In your default constructor, replace
name = "Anonim";
with
strcpy(name, "Anonim");
Your compiler should have warned you about the assignment.
If it didn't, increase the warning level of your compiler.
If it did, start listening to your compiler's warnings.
do not worry. C++ could look a bit scary as first but it is ok when you get into it. First, let's say that all classes it is good to start with upper case letters. Secondly, you have two constructors (default without parameters and one or more with, in our case one). Default consructor you need to declare an array of objects:
Student group[3];
The next important thing is that you then do not need the rest of the constructors in that case.
group[0]=student("Ana",1);
group[1]=student("Alex",2);
group[2]=student("Liam",5);
Remember to include ; at the end of class declaration. To put all the statements and expression throughout your interation within the same loop. Here is what I found as an errors anf fix them. Could probably have more.
class Student
{
private:
char* name;
public:
int nrcrt;
Student()
{
name=new char[7];
strcpy(name, "Anonim");
nrcrt=0;
}
Student( char* n, int n)
{
this->name=new char[7];
strcpy(name, n);
nrcrt=nr;
}
~Student()
{
delete [] name;
}
char* get_name()
{
return this->name;
}
};
int main()
{
Student group[3];
for(int i=0;i<3;i++)
{
if(group.nrcrt[i]!=0)
cout<<group[i].get_name()<<endl;
}
return 0;
}
//Block.h
#pragma once
class Block
{
public:
CRect pos;
int num;
public:
Block(void);
~Block(void);
};
//view class
public:
Block currentState[5]; // stores the current state of the blocks
void CpuzzleView::OnDraw(CDC* pDC)
{
CpuzzleDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
if (!pDoc)
return;
//draw the 4 blocks and put text into them
for(int i=0;i<4;i++)
{
pDC->Rectangle(currentState[i].pos);
// i'm getting an error for this line:
pDC->TextOut(currentState[i].pos.CenterPoint(), currentState[i].num);
}
pDC->TextOut(currentState[i].pos.CenterPoint(), currentState[i].num);
The error says that no instance of overloaded function CDC::TextOutW() matches the argument list . But the prototype for the function is:
CDC::TextOutW(int x, int y, const CString &str )
all i've done is that instead of the 2 points i've directly given the point object returned by CenterPoint() ... shouldn't it work?
That's because you didn't supplied arguments list correctly. Please read compiler error message carefully, it's usually helps to solve the problem.
TextOut(currentState[i].pos.CenterPoint(), currentState[i].num);
In this call you passed CPoint object and int. This is not correct, you need to pass int, int and CString (or const char* and int length).
To fix this you shall do something like this:
CString strState;
strState.Format("%d", currentState[i].num); // Or use atoi()/wtoi() functions
TextOut(currentState[i].pos.CenterPoint().x, currentState[i].pos.CenterPoint().x, strState);