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I have a project which successfully maps a C++ structure to a Fortran equivalent. The structure contains arrays. On the C++ side, the arrays in the structure are represented as:
struct t_res_p {
double *p_length = nullptr;
double *p_diam = nullptr;
}
and on the Fortran side:
type :: t_res_p_ext
sequence
real(8), pointer :: p_length
real(8), pointer :: p_diam
end type t_res_p_ext
Which works fine. I have been recommended to use vectors on the C++ side, but when I use this:
struct t_res_p {
CDArray p_length;
CDArray p_diam;
}
( CDArray is std::vector<double> )
The call from C++ is as follows:
t_res_p p_res;
p_res.p_length.SetSize(ns,0.0);
p_model(&p_res);
And on the Fortran side:
subroutine p_model(p_res) bind(C)
type(t_res_p_ext), target, intent(inout) :: p_res
...
The Fortran structure does not match up. Seems like I need to add .data() somewhere although I'm not sure of the syntax.
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With a VC++ project I have a line:
FXint realIndex = (int)m_cmbDevice->getItemData(indx);
This reads a void pointer value from combo box to an int value. In VC++ it compiles and works well.
Now I have to port this to Linux and there I get a compile error
cpp:514:20: error:
cast from pointer to smaller type 'int' loses information
FXint realIndex = (int)m_cmbDevice->getItemData(indx);
Now I use
std::size_t x = reinterpret_cast<std::size_t>(m_cmbDevice->getItemData(indx));
FXint realIndex = x;
My question is now, is this the right way to go?
reinterpret_cast is dangerous and is typically used only to interpret a pointer as another kind of pointer. You should use static_cast instead
size_t is not necessarily big enough to store a pointer. There are already intptr_t and uintptr_t which are signed and unsigned integer types that are capable of holding a pointer
FXint realIndex = x; still raises a warning if x is wider than FXint, so another cast is necessary to turn off all related warnings
So you need to do this
auto x = reinterpret_cast<uintptr_t>(m_cmbDevice->getItemData(indx));
auto realIndex = (FXint)x;
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I have a container class that contains a char array and an enum/integer indicating what the char array should be cast to. I use it in a callback as so:
void callback(Foo& foo){
switch(foo.type()){
case(1): do( (Bar1*) foo.stuff() );
case(2): do( (Bar2*) foo.stuff() );
case(3): do( (Bar3*) foo.stuff() );
...
}
}
Is there a way to store a mapping from the integer to the type (1, Bar1), (2, Bar2), etc. so that I can clean this switch statement up since it's getting long? Or otherwise, are there any template metaprogramming idioms that can be used in this case?
Store your objects in a std::variant instead of rolling your own using an array of char. Along with std::visit, it supports exactly the use-case you need:
void callback(std::variant<Foo1, Foo2, Foo3>& foo) {
std::visit([](auto& f) { doit(f); }, foo);
}
If you can't use std::variant then a switch is likely the simplest solution given a single known set of types.
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I'm copying a double value to a Void*.How can I do it in C++.
Currently I've tried in C++10
strcpy(Trade->MtTr.MPData->MPTrXML.da,dCou);
Trade->MtTr.MPData->MPTrXML.da-->this is a Void*
dCou is double.
strcpy(Trade->MtTr.MPData->MPTrXML.da,dCou);
I expect the void* should contain the double value.
In actual I'm getting error as:
error C2665: 'strcpy' : none of the 2 overloads could convert all the argument types
while trying to match the argument list '(void *, double)'
If you have a valid reason to be attempting to copy a double to a void*, then one way is to use memcpy:
#include <cstring>
//..
// Assuming Trade->MtTr.MPData->MPTrXML.da is valid:
double dCou;
//...
memcpy(Trade->MtTr.MPData->MPTrXML.da, &dCou, sizeof(double));
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I'm reading Bjarne Stroustrup's book on C++ and he uses things like vector<int> or complex<double>. What does it mean when a data type like int is between a < and > sign?
Tried Googling but it won't recognize my < or >
They're templates.
Function templates are special functions that can operate with generic types. This allows us to create a function template whose functionality can be adapted to more than one type or class without repeating the entire code for each type.
for example:
template <class C>
C add (C a, C b) {
C result = a + b;
return (result);
}
int a = 1;
int b = 2;
add<int>(a, b); //returns 3
float c = 1.5
float d = 0.5
add<float>(c, d) // returns 2.0
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How to cast a Pointer in C++ from class name in String?
Psuedocode:
int * ptr = something;
myStruct ptrstruct = (ClassFromString("myStruct") ptr);
// The class/struct name is passed in as String
Thank you
I'm not a c++ guru but I have two ideas that may help with brainstorming:
May the use of the registry pattern as described here would be of help: Instantiate class from name?
Secondly, following the registry pattern idea you could crate a function for casting e.g. MyClass something = registry.cast("MyClass", ptr);
I am not sure but this must work
Only Void pointer or boost can help, if it happens
thing * p = something; // pointer to object
void * pv = p; // pointer to void
thing * p2 = static_cast<thing *>(pv); // pointer to the same object
Maybe same kind of situation is while returning values from Threads
Overall reflection is not possible in c++.
Its just brainstroming.