XOR encryption using CString

ive been trying to figure out how to do a basic XOR encryption with CStrings but i have errors
im having trouble with the for loop character counter. the code i have to count the number of characters in the CString returns an error from a header file atlsimpstr.h
my code function and error window are as follows:
Attachment 34001Attachment 34003

Where is the bound of the m_output string set and to what value?

How do you know that m_display has at least m_masterkey.GetLength() chars?

Consider

Code:

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m_output = "";
for (int i = 0; i < m_masterkey.GetLength() && i < m_display.GetLength(); i++)
    m_output += m_display.GetAt(i) ^ m_masterkey.GetAt(i);

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