Converting sqlite result to std::string in C++

Question

Im querying a sqlite db, but i need to convert the result (a TEXT attribute) to a C++ std::string. I feel like this should not be hard to accomplish, but im having trouble.

sqlite3_open("sqlite.db", &db);
std::string str = "SELECT something FROM table";
sqlite3_prepare_v2(db,
str.c_str(),
-1,
&m_statement,
0);

sqlite3_step(m_statement);
// OBS: 3rd party printf, cannot use std::cout in this environment
printf("Result1: %s",sqlite3_column_text(m_statement,0)); // OK! Result is printed

string try1 = string(
    reinterpret_cast<const char*>(sqlite3_column_text(m_statement,0))); 
printf("Result2: %s",try1); // null

stringstream ss;
ss << sqlite3_column_text(m_statement,0);
printf("Result3: %s",ss.str()); // null

Doug T. · Accepted Answer · 2012-12-09 01:44:12Z

4

Your problem isn't related to sqlite. When you use %s with printf, printf expects a char * NOT a std::string.

For example change this:

 printf("Result2: %s",try1); // null

to

 printf("Result2: %s",try1.c_str());

and see what happens.

C++ has two primary ways of implementing strings. The legacy C string (char*) and the std::string class found in the C++ standard library. If you deal with C APIs, you'll be dealing mostly with the former. printf and sqlite* are both from C APIs, so expect to deal with char*.

answered Dec 9, 2012 at 1:44

Doug T.

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Alex B · Accepted Answer · 2012-12-09 01:54:27Z

3

The problem is not with sqlite3_column_text.

printf format specifier %s expects a C string (char pointer).

As printf can't inspect what values are passed on the stack, so it has to trust the format specifier. Because the format specifier is %s, it reads an argument as it was a char pointer (it's not), but by coincidence, it's printing the wrong result, rather than crashing.

std::string exposes a function that gets you a read-only corresponding C string, so, for example, your the second case will be:

// Get a C string from std::string
printf("Result2: %s", try1.c_str());

edited Dec 9, 2012 at 1:54

answered Dec 9, 2012 at 1:43

Alex B

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