I programmed a little Application in C++. There is a ListBox in the UI. And I want to use the selected Item of ListBox for an Algorithm where I can use only wstrings.
All in all I have two questions:
-how can I convert my
String^ curItem = listBox2->SelectedItem->ToString();
to a wstring test?
-What means the ^ in the code?
Thanks a lot!
It should be as simple as:
std::wstring result = msclr::interop::marshal_as<std::wstring>(curItem);
You'll also need header files to make that work:
#include <msclr\marshal.h>
#include <msclr\marshal_cppstd.h>
What this marshal_as specialization looks like inside, for the curious:
#include <vcclr.h>
pin_ptr<WCHAR> content = PtrToStringChars(curItem);
std::wstring result(content, curItem->Length);
This works because System::String is stored as wide characters internally. If you wanted a std::string, you'd have to perform Unicode conversion with e.g. WideCharToMultiByte. Convenient that marshal_as handles all the details for you.
I flagged this as a duplicate, but here's the answer on how to get from System.String^ to a std::string.
String^ test = L"I am a .Net string of type System::String";
IntPtr ptrToNativeString = Marshal::StringToHGlobalAnsi(test);
char* nativeString = static_cast<char*>(ptrToNativeString.ToPointer());
The trick is make sure you use Interop and marshalling, because you have to cross the boundary from managed code to non-managed code.
My version is:
Platform::String^ str = L"my text";
std::wstring wstring = str->Data();
With Visual Studio 2015, just do this:
String^ s = "Bonjour!";
C++/CLI
#include <vcclr.h>
pin_ptr<const wchar_t> ptr = PtrToStringChars(s);
C++/CX
const wchart_t* ptr = s->Data();
According to microsoft:
Ref How to: Convert System::String to Standard String
You can convert a String to std::string or std::wstring, without using PtrToStringChars in Vcclr.h.
// convert_system_string.cpp
// compile with: /clr
#include <string>
#include <iostream>
using namespace std;
using namespace System;
void MarshalString ( String ^ s, string& os ) {
using namespace Runtime::InteropServices;
const char* chars =
(const char*)(Marshal::StringToHGlobalAnsi(s)).ToPointer();
os = chars;
Marshal::FreeHGlobal(IntPtr((void*)chars));
}
void MarshalString ( String ^ s, wstring& os ) {
using namespace Runtime::InteropServices;
const wchar_t* chars =
(const wchar_t*)(Marshal::StringToHGlobalUni(s)).ToPointer();
os = chars;
Marshal::FreeHGlobal(IntPtr((void*)chars));
}
int main() {
string a = "test";
wstring b = L"test2";
String ^ c = gcnew String("abcd");
cout << a << endl;
MarshalString(c, a);
c = "efgh";
MarshalString(c, b);
cout << a << endl;
wcout << b << endl;
}
output:
test
abcd
efgh
Related
I have a string with the value 788597.31 and I am converting this value to double but when I print the variable only 788597 is displayed. I have used std::stod(string) and even stringstream but everytime I get the same previous value. Can anybody help me with this?
I want to store this string value in a double varaible.
#include<iostream>
#include<string>
#include<sstream>
using namespace std;
int main()
{
string s="788597.31";
double d=stod(s);
cout<<d<<" ";
stringstream g;
double a;
g<<s; g>>a;
cout<<a;
return 0;
}
The problem is in how you are printing your result, not in the string parsing. This program:
#include <iostream>
using namespace std;
int main() {
cout << 788597.31 << endl;
return 0;
}
also prints 788597.
#include <iostream>
#include <iomanip>
using namespace std;
int main() {
cout << setprecision(10) << 788597.31 << endl;
return 0;
}
prints 788597.31
If you want to see more than the default 6 significant digits your program needs to say so.
For a vector of strings, return the sum of each string's size.
I tried to use accumulate, together with a lambda function (Is it the best way of calculating what I want in 1-line?)
Codes are written in wandbox (https://wandbox.org/permlink/YAqXGiwxuGVZkDPT)
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
using namespace std;
int main() {
vector<string> v = {"abc", "def", "ghi"};
size_t totalSize = accumulate(v.begin(), v.end(), [](string s){return s.size();});
cout << totalSize << endl;
return 0;
}
I expect to get a number (9), however, errors are returned:
/opt/wandbox/gcc-head/include/c++/10.0.0/bits/stl_numeric.h:135:39: note: 'std::__cxx11::basic_string' is not derived from 'const __gnu_cxx::__normal_iterator<_Iterator, _Container>'
135 | __init = _GLIBCXX_MOVE_IF_20(__init) + *__first;
I want to know how to fix my codes? Thanks.
That's because you do not use std::accumulate properly. Namely, you 1) did not specify the initial value and 2) provided unary predicate instead of a binary. Please check the docs.
The proper way to write what you want would be:
#include <iostream>
#include <numeric>
#include <string>
#include <vector>
using namespace std;
int main() {
vector<string> v = {"abc", "def", "ghi"};
size_t totalSize = accumulate(v.begin(), v.end(), 0,
[](size_t sum, const std::string& str){ return sum + str.size(); });
cout << totalSize << endl;
return 0;
}
Both issues are fixed in this code:
0 is specified as initial value, because std::accumulate needs to know where to start, and
The lambda now accepts two parameters: accumulated value, and the next element.
Also note how std::string is passed by const ref into the lambda, while you passed it by value, which was leading to string copy on each invocation, which is not cool
I want to be able to append a constant string to the end of another string in the form of a char*, and then use the resulting string as an argument for open(). Here's what it looks like:
file1.cpp
#include "string.h"
file2 foo;
char* word = "some";
foo.firstWord = word; //I want the file2 class to be able to see "some"
file2.h
#include <fstream>
#include <iostream>
#define SECONDWORD "file.txt"
class file2{
public:
file2();
static char* firstWord;
static char* fullWord;
private:
ofstream stream;
}
file2.cpp
#include "file2.h"
char* file2::firstWord;
char* file2::fullWord;
fullWord = firstWord + SECONDWORD; //so fullWord is now "somefile.txt" ,I know this doesn't work, but basically I am trying to figure out this part
file2::file2(){
stream.open(fullWord);
}
So I am not very well versed in C++, so any help would be appreciated!
C++-style solution might be the following.
#include <string>
char* a = "file";
char* b = ".txt";
...
stream.open((std::string(a) + b).c_str());
What happens here? First, std::string(a) creates a temporary std::string object. Than b value is added to it. At last, c_str() method returns a c-style string which contains a + b.
I found this really nice piece of code that converts a string to a System:String^ as in:
System::String^ rtn = gcnew String(move.c_str()); // 'move' here is the string
I'm passing rtn back to a C# program. Anyways, inside the function where this code exists, I'm passing in a System::String^. I also found some code to convert a System:String^ to a string using the following code:
pin_ptr<const wchar_t> wch = PtrToStringChars(cmd); // 'cmd' here is the System:String
size_t convertedChars = 0;
size_t sizeInBytes = ((cmd->Length + 1) * 2);
errno_t err = 0;
char *ch = (char *)malloc(sizeInBytes);
err = wcstombs_s(&convertedChars,ch, sizeInBytes,wch, sizeInBytes);
Now I can use 'ch' as a string.
This, however, seems to be alot more work than converting the other way using the gcnew. So, at last my question is, is there something out there that will convert a System::String^ to string using a similar fashion as with the gcnew way?
Use VC++'s marshaling library: Overview of Marshaling in C++
#include <msclr/marshal_cppstd.h>
// given System::String^ mstr
std::string nstr = msclr::interop::marshal_as<std::string>(mstr);
this could be useful:
wchar_t *str = "Hi StackOverflow"; //native
String^ mstr= Marshal::PtrToStringAnsi((IntPtr)str); // native to safe managed
wchar_t* A=( wchar_t* )Marshal::StringToHGlobalAnsi(mstr).ToPointer(); // return back to native
don't forget using namespace System::Runtime::InteropServices;
I am writing the code
#include<sstream>
#include<iostream>
using namespace std;
int main(){
strstream temp;
int t =10;
temp>>10;
string tt ="testing"+temp.str();
Have a problem, it does not work at all for the temp variable, just get in result only string testing without 10 in the end?
}
You should use operator<<() instead, temp << 10;.
The problem looks (to me) like a simple typo. You need to replace: temp>>10; with temp<<10;.
As you have included sstream, I think you had the ostringstream class in mind.
ostringstream temp;
int i = 10;
temp << i;
string tt = "testing" + temp.str();
To use strstream, include <strstream>. strstream work with char*, which are C strings. Use ostringstream to work with objects of type basic_string.