CODEWITHSUNDEEP
c++type-conversionimplicit-conversion
SmallChess
SmallChess

Reputation: 8126

Why this Github project converts a string to bool?

This is a very popular C++ project for bioinformatics on Github:

https://github.com/jts/sga/blob/master/src/Util/ClusterReader.cpp

there is a line:

bool good = getline(*m_pReader, line);

I can't compile this line and I don't know why the author did that.

According to the documentation, getline returns a string not bool. Indeed, this is what I get while I tried to compile the project:

ClusterReader.cpp: In member function ‘bool 
ClusterReader::readCluster(ClusterRecord&)’:
ClusterReader.cpp:70:41: error: cannot convert ‘std::basic_istream<char>’ to ‘bool’ in initialization
 bool good = getline(*m_pReader, line);

Why did the C++ code convert a string to bool? How would that be possible?

Upvotes: 3

Views: 111

Answers (2)

songyuanyao
songyuanyao

Reputation: 172984

std::getline doesn't return std::string, but std::basic_istream. For getline(*m_pReader, line);, it just returns *m_pReader.

std::basic_istream could be implicit converted to bool via std::basic_ios::operator bool (since C++11),

Returns true if the stream has no errors and is ready for I/O operations. Specifically, returns !fail().

Before C++11 it could be implicitly converted to void*, which could be converted to bool too.

It seems your compiler failed to perform the implicit conversion, you can use !fail() as a workaround, e.g.

bool good = !getline(*m_pReader, line).fail();

Upvotes: 3

Bobface
Bobface

Reputation: 2952

See this question.

User Loki Astari wrote in his answer:

getline() actually returns a reference to the stream it was used on. When the stream is used in a boolean context this is converted into a unspecified type (C++03) that can be used in a boolean context. In C++11 this was updated and it is converted to bool.

That means you probably do not use an up-to-date compiler (C++03 or even better C++11). If you use g++ or gcc, try adding -std=c++11 to the command.

Upvotes: 2

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