class abc
{
};
int main()
{
std::map<abc, int> m;
abc ob, ob1;
m.insert(std::make_pair(ob, 1));
m.insert(std::make_pair(ob1, 2));
}
Error coming is:
/usr/lib/gcc/i686-redhat-linux/4.7.2/../../../../include/c++/4.7.2/bits/stl_function.h:237:22: note: ‘const abc’ is not derived from ‘const std::multimap<_Key, _Tp, _Compare, _Alloc>’
Does any addition function need be added in class so that it can be made hashable?
Got it. Since std::map sorts when entering a new key in map, so if key is a class object, we must overload class's "<" operator, then it would work fine.
class abc
{
public:
bool operator< (const abc& userObj) const
{
return true;
}
};
int main()
{
std::map<abc, int> m;
abc ob;
abc ob1;
m.insert(std::make_pair(ob, 1));
m.insert(std::make_pair(ob1, 2));
}
Related
In Swift you can pass a type into a function like so:
func foo<T>(_ type: T.Type)
foo(String.self)
Is there any equivalent in Kotlin?
I don't want to specify the type in the generic arguments, because there are multiple generic arguments and it would be annoying to have to specify them all when the compiler can work it out itself except for this one. Also I don't want to use KClass as that requires the type to by Any which I don't want to make a requirement for the type.
you can you class.java.typeName
/**
* Return an informative string for the name of this type.
*
* #return an informative string for the name of this type
* #since 1.8
*/
public String getTypeName() {
if (isArray()) {
try {
Class<?> cl = this;
int dimensions = 0;
do {
dimensions++;
cl = cl.getComponentType();
} while (cl.isArray());
StringBuilder sb = new StringBuilder();
sb.append(cl.getName());
for (int i = 0; i < dimensions; i++) {
sb.append("[]");
}
return sb.toString();
} catch (Throwable e) { /*FALLTHRU*/ }
}
return getName();
}
foo(Int::class.java.typeName)
foo(String::class.java.typeName)
From vala tutorial:
alternative iterator protocol: "T? next_value()"
If the iterator object has a .next_value() function that returns a nullable type then we iterate by calling this function until it returns null.
I write some code, but got an error.
the error message:
error: return type of `Something.next_value' must be nullable
foreach (string s in A) {
^
I don't understand, it is nullable already in my next_value return type.
How to correct this code?
public static void main() {
stdout.printf("hello\n");
var A = new Something<string> ({"aa", "bb"});
foreach (string s in A) {
stdout.puts(s);
stdout.puts ("\n");
}
}
public class Something<T> : Object {
public T[] data;
private int _size;
private int _i = 0;
public Something (owned T[] a){
this.data = a;
this._size = data.length;
}
public Something<T> iterator(){
return this;
}
// error: return type of `Something.next_value' must be nullable
// foreach (string s in A) {
// ^
public T? next_value () {
return _i < _size ? data[_i++] : null;
}
}
You have declared that the generic data type must be nullable, which is correct. So when you pass the specific data type as a type argument make sure it is also nullable. The line:
var A = new Something<string> ({"aa", "bb"});
should be:
var A = new Something<string?> ({"aa", "bb"});
I have this code in CLI
List<Codec^> ^GetCodecs()
{
List<Codec^> ^l = gcnew List<Codec^>;
bool KeepLooping = Encoder_MoveToFirstCodec();
while (KeepLooping)
{
Codec ^codec = gcnew Codec(); // here... and that call encoder_init many times... which call register codec many times... which is a mass...
codec->Name = gcnew String(Encoder_GetCurrentCodecName());
codec->Type = Encoder_GetCurrentCodecType();
char pix_fmts[200]; // array of 200 is probably enough
int actual_pix_fmts_sz = Encoder_GetCurrentCodecPixFmts( pix_fmts , 200 );
for (int i = 0 ; i < actual_pix_fmts_sz ; i++)
{
//copy from pix_fmts to the :List
codec->SupportedPixelFormats->Add(pix_fmts[i]);
}
This is the Encoder_GetCurrentCodecPixFmts function in C:
int Encoder_GetCurrentCodecPixFmts( char *outbuf , int buf_sz )
{
int i=0;
while ( (i<buf_sz) && (codec->pix_fmts[i]!=-1) )
{
outbuf[i] = codec->pix_fmts[i];
i++;
}
return i;
}
This is a new class i did:
#pragma once
using namespace System;
using namespace System::Collections::Generic;
public ref class Codec
{
public:
String^ Name;
int ID; // this is the index
int Type; // this is the type
List<int> ^SupportedPixelFormats;
Codec(void)
{
SupportedPixelFormats = gcnew List<int>;
// do nothing in the constructor;
}
};
Which contain also the: SupportedPixelFormats
The constructor in this new class should be empty but i needed somewhere to make an instance for the List make a NEW for the List.
Now in the C++ i need to transfer from pix_fmts char array to codec->Supported
Or to copy from pix_fmts to the :List
So i did as above:
codec->SupportedPixelFormats->Add(pix_fmts[i]);
But i'm not sure if this the meaning of copy.
Is that right what i did ?
It works, it's a kind of a deep copy. What makes you think it doesn't work? Do the results turn out wrong? If they do, put a breakpoint in there and try to get what is wrong.
Instead of copying one by one perhaps you can use the Enumerable::ToList extension method.
I hope this helped you.
Suppose I have two classes:
class1 {
int m_i;
std::string m_s;
};
class2 {
int m_i2;
class1 *m_ptr;
};
Now, I want to send a class2 variable over network, and want to use any of the libraries that does serialization.(Protocol-buffers, Thrift, MessagePack..)
Which one can I use?(note the class1* m_ptr)
You could use thrift for this.
the definition would look something like
struct class1 {
1: required i32 m_i;
2: required string m_s;
}
struct class2 {
1: required i32 m_i2;
2: optional class1 m_ptr;
}
You would like to read this excellent guide
http://diwakergupta.github.com/thrift-missing-guide/
and to get clarity on concern about the "pointer" issue that you mentioned in the question,read the section on "How are nested structs initialized?" in the above guide.
Using google protocol buffers, you would need a .proto file (say test.proto) like:
package serialisation; // puts this in namespace serialisation
message class1 {
required int32 m_i = 1;
required bytes m_s = 2;
}
message class2 {
required int32 m_i2 = 1;
optional class1 m_ptr = 2;
}
Using C++, once you run the protoc compiler against this, you end up with test.pb.cc and test.pb.h
You can then use these like:
#include <string>
#include "test.pb.h"
struct class1 {
int m_i;
std::string m_s;
};
struct class2 {
int m_i2;
class1 *m_ptr;
};
int main() {
class2 second_class;
second_class.m_i2 = 2;
second_class.m_ptr = new class1;
second_class.m_ptr->m_i = 1;
second_class.m_ptr->m_s = "one";
// Serialise class 2
serialisation::class2 serialisable_second_class;
serialisable_second_class.set_m_i2(second_class.m_i2);
if (second_class.m_ptr) {
serialisation::class1* serialisable_first_class = serialisable_second_class.mutable_m_ptr();
serialisable_first_class->set_m_i(second_class.m_ptr->m_i);
serialisable_first_class->set_m_s(second_class.m_ptr->m_s);
}
std::string serialised(serialisable_second_class.SerializeAsString());
// Parse class 2
serialisation::class2 parsed_second_class;
parsed_second_class.ParseFromString(serialised);
class2 retrieved_second_class;
retrieved_second_class.m_i2 = parsed_second_class.m_i2();
if (parsed_second_class.has_m_ptr()) {
retrieved_second_class.m_ptr = new class1;
retrieved_second_class.m_ptr->m_i = parsed_second_class.m_ptr().m_i();
retrieved_second_class.m_ptr->m_s = parsed_second_class.m_ptr().m_s();
} else {
retrieved_second_class.m_ptr = nullptr;
}
return 0;
}
Note, for the sake of brevity I'm not doing any error checking or exception handling here - this would be needed in production code. I'm also not managing the lifetime of the class1 pointer.
Does anyone have a working, step-by-step example of how to implement IEnumerable and IEnumerator in C++/CLI? Alternatively, does someone know how to fix the following code from MS Connect which does not compile in Visual Studio 2005?
http://connect.microsoft.com/VisualStudio/feedback/details/101089/how-to-implement-ienumerable-t-and-ienumerable-c-cli
using namespace System;
using namespace System::Collections::Generic;
generic <class T>
public ref struct MyArray : public IEnumerable<T>
{
MyArray( array<T>^ d )
{
data = d;
}
ref struct enumerator : IEnumerator<T>
{
enumerator( MyArray^ myArr )
{
colInst = myArr;
currentIndex = -1;
}
bool MoveNext()
{
if( currentIndex < colInst->data->Length - 1 )
{
currentIndex++;
return true;
}
return false;
}
property T Current
{
T get()
{
return colInst->data[currentIndex];
}
};
// This is required as IEnumerator<T> also implements IEnumerator
property Object^ Current2
{
virtual Object^ get() new sealed = System::Collections::IEnumerator::Current::get
{
return colInst->data[currentIndex];
}
};
void Reset() {}
~enumerator() {}
MyArray^ colInst;
int currentIndex;
};
array<T>^ data;
IEnumerator<T>^ GetEnumerator()
{
return gcnew enumerator(this);
}
virtual System::Collections::IEnumerator^ GetEnumerator2() new sealed = System::Collections::IEnumerable::GetEnumerator
{
return gcnew enumerator(this);
}
};
int main()
{
int retval = 0;
MyArray<int>^ col = gcnew MyArray<int>( gcnew array<int>{10, 20, 30 } );
for each( Object^ c in col )
{
retval += (int)c;
}
retval -= 10 + 20 + 30;
Console::WriteLine("Return Code: {0}", retval );
return retval;
}
The compiler is unable to find the enumerator method implementations:
error C3766: 'MyArray<T>::enumerator' must provide an implementation for the interface method 'bool System::Collections::IEnumerator::MoveNext(void)' c:\Projects\VCNET\2005\IEnumerable\IEnumerable\IEnumerable.cpp 55
error C3766: 'MyArray<T>::enumerator' must provide an implementation for the interface method 'void System::Collections::IEnumerator::Reset(void)' c:\Projects\VCNET\2005\IEnumerable\IEnumerable\IEnumerable.cpp 55
error C3766: 'MyArray<T>::enumerator' must provide an implementation for the interface method 'T System::Collections::Generic::IEnumerator<T>::Current::get(void)' c:\Projects\VCNET\2005\IEnumerable\IEnumerable\IEnumerable.cpp 55
error C3766: 'MyArray<T>' must provide an implementation for the interface method 'System::Collections::Generic::IEnumerator<T> ^System::Collections::Generic::IEnumerable<T>::GetEnumerator(void)' c:\Projects\VCNET\2005\IEnumerable\IEnumerable\IEnumerable.cpp 68
This compiles for me without a single warning (on VS2010):
using namespace System;
using namespace System::Collections::Generic;
generic <class T>
public ref struct MyArray : public IEnumerable<T>
{
MyArray( array<T>^ d )
{
data = d;
}
ref struct enumerator : IEnumerator<T>
{
enumerator( MyArray^ myArr )
{
colInst = myArr;
currentIndex = -1;
}
virtual bool MoveNext() = IEnumerator<T>::MoveNext
{
if( currentIndex < colInst->data->Length - 1 )
{
currentIndex++;
return true;
}
return false;
}
property T Current
{
virtual T get() = IEnumerator<T>::Current::get
{
return colInst->data[currentIndex];
}
};
// This is required as IEnumerator<T> also implements IEnumerator
property Object^ Current2
{
virtual Object^ get() = System::Collections::IEnumerator::Current::get
{
return colInst->data[currentIndex];
}
};
virtual void Reset() = IEnumerator<T>::Reset {}
~enumerator() {}
MyArray^ colInst;
int currentIndex;
};
array<T>^ data;
virtual IEnumerator<T>^ GetEnumerator()
{
return gcnew enumerator(this);
}
virtual System::Collections::IEnumerator^ GetEnumerator2() = System::Collections::IEnumerable::GetEnumerator
{
return gcnew enumerator(this);
}
};
int main()
{
int retval = 0;
MyArray<int>^ col = gcnew MyArray<int>( gcnew array<int>{10, 20, 30 } );
for each( Object^ c in col )
{
retval += (int)c;
}
retval -= 10 + 20 + 30;
Console::WriteLine("Return Code: {0}", retval );
return retval;
}