I am using std::list's predicate to update the list based on predicate. But calling in the OnInitDialog() throws compilation error. My code is as follows:
The below is .h:
class CDlgWindow : public CDialog
{
private:
bool single_digit (const int &value);
int _days;
}
The below is .cpp:
CDlgWindow::CDlgWindow(CWnd* pParent, CString strInfo, int days) //ctor
{
_days = days;
//_strInfo = strInfo
}
bool CDlgWindow::single_digit(const int& value)
{
return (value >= _days);
}
BOOL CDlgWindow::OnInitDialog()
{
CDialog::OnInitDialog();
CenterWindow();
.
.
.
int numArr[] = {10,20,30,40};
int size = sizeof(numArr)/sizeof(numArr[0]);
std::list<int> numList (numArr, numArr+size);
numList.remove_if(single_digit); //Error C3867 here!
.
.
}
Complete error message:
Error C3867 function call missing argument list, use '&CDlgWindow::single_digit' to create a pointer to member.
I am trying to understand the functors concept. As I checked in C++11, we have lambdas for easier implementation. Please guide me to understand more on this issue. Thanks!
std::list's remove_if member needs a unary predicate (p) that operates on values (v). The expression p(v) must be valid. Which it isn't if p is a non-static class member (see repro).
There are two options:
Make the predicate (single_digit) a static class member:
class CDlgWindow : public CDialog
{
private:
static bool single_digit (const int &value);
// ...
}
Make the predicate a free function:
bool single_digit(int const& value) {
static int days_ = ...;
return (value >= days_);
}
If you go with option 1 you will have to make _days static as well, since a static member function cannot access non-static instance data. If _days is a compile-time constant, make sure to mark it const as well. That'll open up some compiler optimizations.
This is all hoping that things haven't significantly changed between C++98 and C++11. It's hard to find a C++98 compiler to verify this.
For example, consider the following C# code:
interface IBase { void f(int); }
interface IDerived : IBase { /* inherits f from IBase */ }
...
void SomeFunction()
{
IDerived o = ...;
o.f(5);
}
I know how to get a MethodDefinition object corresponding to SomeFunction.
I can then loop through MethodDefinition.Instructions:
var methodDef = GetMethodDefinitionOfSomeFunction();
foreach (var instruction in methodDef.Body.Instructions)
{
switch (instruction.Operand)
{
case MethodReference mr:
...
break;
}
yield return memberRef;
}
And this way I can find out that the method SomeFunction calls the function IBase.f
Now I would like to know the declared type of the object on which the function f is called, i.e. the declared type of o.
Inspecting mr.DeclaringType does not help, because it returns IBase.
This is what I have so far:
TypeReference typeRef = null;
if (instruction.OpCode == OpCodes.Callvirt)
{
// Identify the type of the object on which the call is being made.
var objInstruction = instruction;
if (instruction.Previous.OpCode == OpCodes.Tail)
{
objInstruction = instruction.Previous;
}
for (int i = mr.Parameters.Count; i >= 0; --i)
{
objInstruction = objInstruction.Previous;
}
if (objInstruction.OpCode == OpCodes.Ldloc_0 ||
objInstruction.OpCode == OpCodes.Ldloc_1 ||
objInstruction.OpCode == OpCodes.Ldloc_2 ||
objInstruction.OpCode == OpCodes.Ldloc_3)
{
var localIndex = objInstruction.OpCode.Op2 - OpCodes.Ldloc_0.Op2;
typeRef = locals[localIndex].VariableType;
}
else
{
switch (objInstruction.Operand)
{
case FieldDefinition fd:
typeRef = fd.DeclaringType;
break;
case VariableDefinition vd:
typeRef = vd.VariableType;
break;
}
}
}
where locals is methodDef.Body.Variables
But this is, of course, not enough, because the arguments to a function can be calls to other functions, like in f(g("hello")). It looks like the case above where I inspect previous instructions must repeat the actions of the virtual machine when it actually executes the code. I do not execute it, of course, but I need to recognize function calls and replace them and their arguments with their respective returns (even if placeholders). It looks like a major pain.
Is there a simpler way? Maybe there is something built-in already?
I am not aware of an easy way to achieve this.
The "easiest" way I can think of is to walk the stack and find where the reference used as the target of the call is pushed.
Basically, starting from the call instruction go back one instruction at a time taking into account how each one affects the stack; this way you can find the exact instruction that pushes the reference used as the target of the call (a long time ago I wrote something like that; you can use the code at https://github.com/lytico/db4o/blob/master/db4o.net/Db4oTool/Db4oTool/Core/StackAnalyzer.cs as inspiration).
You'll need also to consider scenarios in which the pushed reference is produced through a method/property; for example, SomeFunction().f(5). In this case you may need to evaluate that method to find out the actual type returned.
Keep in mind that you'll need to handle a lot of different cases; for example, imagine the code bellow:
class Utils
{
public static T Instantiate<T>() where T : new() => new T();
}
class SomeType
{
public void F(int i) {}
}
class Usage
{
static void Main()
{
var o = Utils.Instantiate<SomeType>();
o.F(1);
}
}
while walking the stack you'll find that o is the target of the method call; then you'll evaluate Instantiate<T>() method and will find that it returns new T() and knowing that T is SomeType in this case, that is the type you're looking for.
So the answer of Vagaus helped me come up with a working implementation.
I published it on github - https://github.com/MarkKharitonov/MonoCecilExtensions
Included many unit tests, but I am sure I missed some cases.
I was wondering how I could change the code below such the bmBc is computed at compile time . The one below works for runtime but it is not ideal since I need to know the bmBc table at compile-time . I could appreciate advice on how I could improve on this.
import std.conv:to;
import std.stdio;
int [string] bmBc;
immutable string pattern = "GCAGAGAG";
const int size = to!int(pattern.length);
struct king {
void calculatebmBc(int i)()
{
static if ( i < size -1 )
bmBc[to!string(pattern[i])]=to!int(size-i-1);
// bmBc[pattern[i]] ~= i-1;
calculatebmBc!(i+1)();
}
void calculatebmBc(int i: size-1)() {
}
}
void main(){
king myKing;
const int start = 0;
myKing.calculatebmBc!(start)();
//1. enum bmBcTable = bmBc;
}
The variables bmBc and bmh can't be read at compile time because you define them as regular runtime variables.
You need to define them as enums, or possibly immutable, to read them at compile time, but that also means that you cannot modify them after initialization. You need to refactor your code to return values instead of using out parameters.
Alternatively, you can initialize them at runtime inside of a module constructor.
I have a std::vector of this struct:
struct MS
{
double aT;
double bT;
double cT;
};
which I want to use std::sort on as well as std::lower_bound/equal_range etc...
I need to be able to sort it and look it up on either of the first two elements of the struct. So at the moment I have this:
class MSaTLess
{
public:
bool operator() (const MS &lhs, const MS &rhs) const
{
return TLess(lhs.aT, rhs.aT);
}
bool operator() (const MS &lhs, const double d) const
{
return TLess(lhs.aT, d);
}
bool operator() (const double d, const MS &rhs) const
{
return TLess(d, rhs.aT);
}
private:
bool TLess(const double& d1, const double& d2) const
{
return d1 < d2;
}
};
class MSbTLess
{
public:
bool operator() (const MS &lhs, const MS &rhs) const
{
return TLess(lhs.bT, rhs.bT);
}
bool operator() (const MS &lhs, const double d) const
{
return TLess(lhs.bT, d);
}
bool operator() (const double d, const MS &rhs) const
{
return TLess(d, rhs.bT);
}
private:
bool TLess(const double& d1, const double& d2) const
{
return d1 < d2;
}
};
This allows me to call both std::sort and std::lower_bound with MSaTLess() to sort/lookup based on the aT element and with MSbTLess() to sort/lookup based on the bT element.
I'd like to get away from the functors and use C++0x lambdas instead. For sort that is relatively straightforward as the lambda will take two objects of type MS as arguments.
What about for the lower_bound and other binary search lookup algorithms though? They need to be able to call a comparator with (MS, double) arguments and also the reverse, (double, MS), right? How can I best provide these with a lambda in a call to lower_bound? I know I could create an MS dummy object with the required key value being searched for and then use the same lambda as with std::sort but is there a way to do it without using dummy objects?
It's a little awkward, but if you check the definitions of lower_bound and upper_bound from the standard, you'll see that the definition of lower_bound puts the dereferenced iterator as the first parameter of the comparison (and the value second), whereas upper_bound puts the dereferenced iterator second (and the value first).
So, I haven't tested this but I think you'd want:
std::lower_bound(vec.begin(), vec.end(), 3.142, [](const MS &lhs, double rhs) {
return lhs.aT < rhs;
});
and
std::upper_bound(vec.begin(), vec.end(), 3.142, [](double lhs, const MS &rhs) {
return lhs < rhs.aT;
});
This is pretty nasty, and without looking up a few more things I'm not sure you're actually entitled to assume that the implementation uses the comparator only in the way it's described in the text - that's a definition of the result, not the means to get there. It also doesn't help with binary_search or equal_range.
It's not explicitly stated in 25.3.3.1 that the iterator's value type must be convertible to T, but it's sort of implied by the fact that the requirement for the algorithm is that T (in this case, double) must be LessThanComparable, not that T must be comparable to the value type of the iterator in any particular order.
So I think it's better just to always use a lambda (or functor) that compares two MS structs, and instead of passing a double as a value, pass a dummy MS with the correct field set to the value you're looking for:
std::upper_bound(vec.begin(), vec.end(), MS(3.142,0,0), [](const MS &lhs, const MS &rhs) {
return lhs.aT < rhs.aT;
});
If you don't want to give MS a constructor (because you want it to be POD), then you can write a function to create your MS object:
MS findA(double d) {
MS result = {d, 0, 0};
return result;
}
MS findB(double d) {
MS result = {0, d, 0};
return result;
}
Really, now that there are lambdas, for this job we want a version of binary search that takes a unary "comparator":
double d = something();
unary_upper_bound(vec.begin(), vec.end(), [d](const MS &rhs) {
return d < rhs.aT;
});
C++0x doesn't provide it, though.
The algorithms std::sort, std::lower_bound, and std::binary_search take a predicate that compares two elements of the container. Any lambda that compares two MS objects and returns true when they are in order should work for all three algorithms.
Not directly relevant to what you're saying about lambdas, but this might be an idea for using the binary search functions:
#include <iostream>
#include <algorithm>
#include <vector>
struct MS
{
double aT;
double bT;
double cT;
MS(double a, double b, double c) : aT(a), bT(b), cT(c) {}
};
// template parameter is a data member of MS, of type double
template <double MS::*F>
struct Find {
double d;
Find(double d) : d(d) {}
};
template <double MS::*F>
bool operator<(const Find<F> &lhs, const Find<F> &rhs) {
return lhs.d < rhs.d;
}
template <double MS::*F>
bool operator<(const Find<F> &lhs, const MS &rhs) {
return lhs.d < rhs.*F;
}
template <double MS::*F>
bool operator<(const MS &lhs, const Find<F> &rhs) {
return lhs.*F < rhs.d;
}
int main() {
std::cout << (Find<&MS::bT>(1) < Find<&MS::bT>(2)) << "\n";
std::cout << (Find<&MS::bT>(1) < MS(1,0,0)) << "\n";
std::cout << (MS(1,0,0) < Find<&MS::bT>(1)) << "\n";
std::vector<MS> vec;
vec.push_back(MS(1,0,0));
vec.push_back(MS(0,1,0));
std::lower_bound(vec.begin(), vec.end(), Find<&MS::bT>(0.5));
std::upper_bound(vec.begin(), vec.end(), Find<&MS::bT>(0.5));
}
Basically, by using Find as the value, we don't have to supply a comparator, because Find compares to MS using the field that we specify. This is the same kind of thing as the answer you saw over here: how to sort STL vector, but using the value rather than the comparator as in that case. Not sure if it'd be all that great to use, but it might be, since it specifies the value to search for and the field to search in a single short expression.
I had the same problem for std::equal_range and came up with an alternative solution.
I have a collection of pointers to objects sorted on a type field. I need to find the find the range of objects for a given type.
const auto range = std::equal_range (next, blocks.end(), nullptr,
[type] (Object* o1, Object* o2)
{
return (o1 ? o1->Type() : type) < (o2 ? o2->Type() : type);
});
Although it is less efficient than a dedicated predicate as it introduces an unnecessary nullptr test for each object in my collection, it does provide an interesting alternative.
As an aside, when I do use a class as in your example, I tend to do the following. As well as being shorter, this allows me to add additional types with only 1 function per type rather then 4 operators per type.
class MSbTLess
{
private:
static inline const double& value (const MS& val)
{
return val.bT;
}
static inline const double& value (const double& val)
{
return val;
}
public:
template <typename T1, typename T2>
bool operator() (const T1& lhs, const T2& rhs) const
{
return value (t1) < value (t2);
}
};
In the definition of lower_bound and other STL Algorithms the Compare function is such that the first type must match that of the Forward Iterator and the second type must match that of T (i.e., of the value).
template< class ForwardIt, class T, class Compare >
ForwardIt lower_bound( ForwardIt first, ForwardIt last, const T& value, Compare comp );
So one indeed can compare things from different objects (doing what the other response called an Unary Comparator). In C++11 :
vector<MS> v = SomeSortedVectorofMSByFieldaT();
double a_key;
auto it = std::lower_bound(v.begin(),
v.end(),
a_key,
[]{const MS& m, const double& a) {
m.aT < a;
});
And this can be used with other STL algorithm functions as well.
I would like to see a simple example of how to override stdext::hash_compare properly, in order to define a new hash function and comparison operator for my own user-defined type. I'm using Visual C++ (2008).
This is how you can do it
class MyClass_Hasher {
const size_t bucket_size = 10; // mean bucket size that the container should try not to exceed
const size_t min_buckets = (1 << 10); // minimum number of buckets, power of 2, >0
MyClass_Hasher() {
// should be default-constructible
}
size_t operator()(const MyClass &key) {
size_t hash_value;
// do fancy stuff here with hash_value
// to create the hash value. There's no specific
// requirement on the value.
return hash_value;
}
bool operator()(const MyClass &left, const MyClass &right) {
// this should implement a total ordering on MyClass, that is
// it should return true if "left" precedes "right" in the ordering
}
};
Then, you can just use
stdext::hash_map my_map<MyClass, MyValue, MyClass_Hasher>
Here you go, example from MSDN
I prefer using a non-member function.
The method expained in the Boost documentation article Extending boost::hash for a custom data type seems to work.