How to get input into a messageBox - c++

Question

I'm fairly new to C++. I tried practicing my code but I hit an error I'm not sure how to fix:
Error: expression cannot be used as a function
switch (message)
{
case WM_CREATE:
TextBox = CreateWindow("EDIT",
"",
WS_VISIBLE|WS_CHILD|WS_BORDER,
10,10,80,20,
hwnd,NULL,NULL,NULL);
BUTTON = CreateWindow("BUTTON",
"GO",
WS_VISIBLE|WS_CHILD|WS_BORDER,
100,10,70,20,
hwnd,(HMENU)1,NULL,NULL);
break;
case WM_COMMAND:
case 1:
int gwtstat=0;
//char *t=&textSaved[0];
gwtstat=GetWindowText(TextBox,&textSaved[0], 50);
::MessageBox(
hwnd,
"I am" ()textSaved "and i am Beautiful!",
"smile",
MB_YESNO);

Answer 1

The issue is here:
"I am" ()
The expression "I am" is of type char const[5] which doesn't support function call syntax. What you intended to do is probably string interpolation. For that you can use std::format starting with C++20.
The following solves the immediate issue:
::MessageBox(
hwnd,
std::format("I am {} and i am Beautiful!", textSaved).c_str(),
"smile",
MB_YESNO);
There are lots of other issues with the code, some less fatal than others:
Use of the ANSI API as opposed to the Unicode API. The latter is strongly recommended, as it allows use of the entire set of Unicode code points (and has less overhead)
Assigning terribly low IDs to controls (0 and 1, respectively) (see Why do dialog editors start assigning control IDs with 100? for details)
Defining a variable in a case label; your compiler warned you about this
Failure to extract the control ID from the WM_COMMAND message handler
A more appropriate implementation could look like this:
// Declare symbolic constants for control IDs
constexpr int TextBoxId = 100;
constexpr int GoButtonId = 101;
// ...
switch (message)
{
case WM_CREATE:
TextBox = ::CreateWindowW(L"EDIT",
L"",
WS_VISIBLE|WS_CHILD|WS_BORDER,
10,10,80,20,
hwnd,(HMENU)TextBoxId,NULL,NULL);
BUTTON = ::CreateWindowW(L"BUTTON",
L"GO",
WS_VISIBLE|WS_CHILD|WS_BORDER,
100,10,70,20,
hwnd,(HMENU)GoButtonId,NULL,NULL);
break;
case WM_COMMAND:
// Evaluate control ID of the sender
switch(LOWORD(wParam)) {
case GoButtonId: {
// ^ Introduce new scope for automatic
// lifetime management of local variables
// Retrieve text from text box
size_t const length = ::GetWindowTextLengthW(TextBox);
std::wstring text(length, L'\0');
::GetWindowTextW(TextBox, text.data(), text.length() + 1);
::MessageBoxW(
hwnd,
std::format(L"I am {} and i am Beautiful!", text).c_str(),
L"smile",
MB_YESNO);
// ...
}
break;
While this implementation reads the edit control's contents into a std::wstring variable the following will work as well, and doesn't require C++20 (compiles with C++98):
::MessageBoxW(
hwnd,
(L"I am " + text + L" and i am Beautiful!").c_str(),
L"smile",
MB_YESNO);