const char* not being interpreted correctly by C++ to Excel library

Question

I am trying to use libXl to output text from a C++ program to an Excel file.

The problem is coming with this library function:

bool writeStr(int row, int col, const wchar_t* value, Format* format = 0)

Writes a string into cell with specified format. If format equals 0 then format is ignored. String is copied internally and can be destroyed after call this method. Returns false if error occurs. Get error info with Book::errorMessage().

If I give input as a string literal like "Hello World" it is displayed correctly. However, if I try to give input as a variable of type const char*, it displays garbage.

Following is my code. MyCompany::company is a QString .

const char* companyName = MyCompany::company.toStdString().c_str();
sheet->writeStr(4, 0, companyName, companyFormat);

Can anybody tell me what's going on? How can I display a variable string using this library?

Thanks for your help.

Answer 1

 const char* companyName = MyCompany::company.toStdString().c_str();

The problem here is that the buffer companyName points to becomes invalid immediately after the end of this statement, as c_str() becomes invalid when the temporary std::string returned by toStdString() is destroyed. You can solve this by keeping the std::string long enough:

const std::string companyName = MyCompany::company.toStdString(); //survives until the end of the block
sheet->writeStr(4, 0, companyName.c_str(), companyFormat);

I'd suggest to not convert via std::string though but convert to some well-defined encoding instead. For example UTF-8:

const QByteArray companyName = MyCompany::company.toUtf8();
sheet->writeStr(4, 0, companyName.constData(), companyFormat);

(Note that const char* companyName = MyCompany::company.toUtf8().constData() would have the same problem as your code above).