C++ MessageBox character array - c++

Question

I am having difficulties using MessageBox function with variables
I have
int main(int argc, char* argv[])
{
char* filename = argv[0];
DWORD length = strlen(filename);
MessageBox(0, TEXT("filename text"), TEXT("length text"), 0); // Works
}
But I want to output variables filename and length as:
MessageBox(0, filename, length, 0); -- compiler error
Function MessageBox has syntax:
int WINAPI MessageBox(
_In_opt_ HWND hWnd,
_In_opt_ LPCTSTR lpText,
_In_opt_ LPCTSTR lpCaption,
_In_ UINT uType
);
I tried using
MessageBox(0, (LPCWSTR)filename, (LPCWSTR)length, 0);
but the output is in some kind of hieroglyphs.

Answer 1

The variable length is not a string, and only strings can be used. It doesn't help that you try to cast it to a char* as then the value of length will be taken as a pointer to the string which will cause undefined behavior.
For C++, you can use e.g. std::to_string to convert non-string values to strings, like
MessageBox(0, filename, std::to_string(length).c_str(), 0);
Note that you must use the c_str function to get a char*.
If you don't have std::to_string then you can use e.g. std::istringstream instead:
std::istringstream is;
is << length;
MessageBox(0, filename, is.str().c_str(), 0);
If you want a more old-style C solution, then there's snprintf (or _snprintf in Visual Studio):
char length_string[20];
_snprintf(length_string, sizeof(length_string), "%ld", length);
MessageBox(0, filename, length_string, 0);

Answer 2

With a C++ win32 project in VS2015 a char array displays in a MessageBox with this code. Include header atlstr.h
// open a file in read mode.
ifstream myInfile;
myInfile.open("C:\\Users\\Desktop\\CodeOnDesktop\\myTrialMessageBox.txt");
if (myInfile.fail())
{
MessageBox(NULL,
L"We have an error trying to open the file myTrialMessageBox.txt",
L"Opening a file.",
MB_ICONERROR);
}
char data[200];
// Read the data from the file and display it.
//infile >> data; // Only gets the first word.
myInfile.getline(data, 100);
//To use CString, include the atlstr.h header.
// Cast array called data to a CString to enable use as MessageBox parameter.
CString cdata = (CString)data;
// or CString cdata = CString(_T("A string"));
MessageBox(NULL,
cdata,
L"Opening a file.",
MB_ICONERROR);
// close the opened file.
myInfile.close();