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C++ compiler does not check if a method exists in template class

Storing C++ template function definitions in a .CPP fileUse 'class' or 'typename' for template parameters?Is it possible to write a template to check for a function's existence?Why does C++ compilation take so long?Difference of keywords 'typename' and 'class' in templates?C++ template typedefC++11 introduced a standardized memory model. What does it mean? And how is it going to affect C++ programming?“Undefined reference to” template class constructorC++ Basic Template Class CompilationC++ Abstract Base Template Class Non-Void Method

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6

0

I came across with the following program in C++:

template <class T>
class Val 
protected:
 T x0, x;
public:
 Val(T t = 1) : x0(t), x(1) 
 T val() return x; 
 void promote() this->promote_value(); 
;

For some reason Val<int>(4).val(); works fine even though there is no method promote_value(). I tried to remove the templates:

class OtherVal 
protected:
 int x0, x;
public:
 OtherVal (int t = 1) : x0(t), x(1) 
 int val() return x; 
 void promote() this->promote_value(); 
;

But now I get an error:

error: ‘class OtherVal’ has no member named ‘promote_value’; did you mean ‘promote’?

Why does C++ behave like this?

c++ templates

share|improve this question

edited 7 hours ago

Gašper Čefarin

5088 bronze badges

asked 8 hours ago

vesiivesii

38311 silver badge1212 bronze badges

add a comment | 

6

0

I came across with the following program in C++:

template <class T>
class Val 
protected:
 T x0, x;
public:
 Val(T t = 1) : x0(t), x(1) 
 T val() return x; 
 void promote() this->promote_value(); 
;

For some reason Val<int>(4).val(); works fine even though there is no method promote_value(). I tried to remove the templates:

class OtherVal 
protected:
 int x0, x;
public:
 OtherVal (int t = 1) : x0(t), x(1) 
 int val() return x; 
 void promote() this->promote_value(); 
;

But now I get an error:

error: ‘class OtherVal’ has no member named ‘promote_value’; did you mean ‘promote’?

Why does C++ behave like this?

c++ templates

share|improve this question

edited 7 hours ago

Gašper Čefarin

5088 bronze badges

asked 8 hours ago

vesiivesii

38311 silver badge1212 bronze badges

add a comment | 

I came across with the following program in C++:

template <class T>
class Val 
protected:
 T x0, x;
public:
 Val(T t = 1) : x0(t), x(1) 
 T val() return x; 
 void promote() this->promote_value(); 
;

For some reason Val<int>(4).val(); works fine even though there is no method promote_value(). I tried to remove the templates:

class OtherVal 
protected:
 int x0, x;
public:
 OtherVal (int t = 1) : x0(t), x(1) 
 int val() return x; 
 void promote() this->promote_value(); 
;

But now I get an error:

error: ‘class OtherVal’ has no member named ‘promote_value’; did you mean ‘promote’?

Why does C++ behave like this?

c++ templates

share|improve this question

edited 7 hours ago

Gašper Čefarin

5088 bronze badges

asked 8 hours ago

vesiivesii

38311 silver badge1212 bronze badges

template <class T>
class Val 
protected:
 T x0, x;
public:
 Val(T t = 1) : x0(t), x(1) 
 T val() return x; 
 void promote() this->promote_value(); 
;

class OtherVal 
protected:
 int x0, x;
public:
 OtherVal (int t = 1) : x0(t), x(1) 
 int val() return x; 
 void promote() this->promote_value(); 
;

share|improve this question

edited 7 hours ago

Gašper Čefarin

5088 bronze badges

asked 8 hours ago

vesiivesii

38311 silver badge1212 bronze badges

share|improve this question

edited 7 hours ago

Gašper Čefarin

5088 bronze badges

asked 8 hours ago

vesiivesii

38311 silver badge1212 bronze badges

edited 7 hours ago

Gašper Čefarin

5088 bronze badges

edited 7 hours ago

Gašper Čefarin

5088 bronze badges

asked 8 hours ago

vesiivesii

38311 silver badge1212 bronze badges

asked 8 hours ago

vesiivesii

38311 silver badge1212 bronze badges

2 Answers
2

active

oldest

votes

7

Template class methods are not instantiated until they are used. Once you try calling promote() or even get its address like this &Val<int>::promote then you'll get an error.

From the C++ standard:

§ 17.8.1.10 An implementation shall not implicitly instantiate a function
template, a variable template, a member template, a non-virtual member
function, a member class, a static data member of a class template,
or a substatement of a constexpr if statement (9.4.1), unless such
instantiation is required.

share|improve this answer

edited 6 hours ago

alter igel

4,26511 gold badge1414 silver badges3333 bronze badges

answered 8 hours ago

r3mus n0xr3mus n0x

3,72211 gold badge33 silver badges2525 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you. Are there any checks at all that the compiler does on promote()? I guess syntax errors.
  
  – vesii
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vesil, AFAIK the compiler is obligated by the standard to parse the non-instantiated method definition and check it for correctness as much as it can without knowing the T. However the MSVC is known for violating that requirement and not bothering to check anything thus allowing a complete nonsense in method definition which will only be discovered when the method is actually used.
  
  – r3mus n0x
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vesil, the nonsense example: godbolt.org/z/q0Xvvz :)
  
  – r3mus n0x
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So if the method promote() is virtual it will fail in compilation?
  
  – vesii
  3 hours ago
  
  

  
  
  
  

add a comment | 

5

Templates have always worked this way, principally to facilitate their use.

Because Val<int>(4).val(); doesn't call promote, that function is not compiled for your particular instantiation of that template so the compiler does not issue a diagnostic.

Many metaprogramming techniques depend on this behaviour.

share|improve this answer

answered 8 hours ago

BathshebaBathsheba

189k2727 gold badges282282 silver badges400400 bronze badges

add a comment | 

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7

Template class methods are not instantiated until they are used. Once you try calling promote() or even get its address like this &Val<int>::promote then you'll get an error.

From the C++ standard:

§ 17.8.1.10 An implementation shall not implicitly instantiate a function
template, a variable template, a member template, a non-virtual member
function, a member class, a static data member of a class template,
or a substatement of a constexpr if statement (9.4.1), unless such
instantiation is required.

share|improve this answer

edited 6 hours ago

alter igel

4,26511 gold badge1414 silver badges3333 bronze badges

answered 8 hours ago

r3mus n0xr3mus n0x

3,72211 gold badge33 silver badges2525 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you. Are there any checks at all that the compiler does on promote()? I guess syntax errors.
  
  – vesii
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vesil, AFAIK the compiler is obligated by the standard to parse the non-instantiated method definition and check it for correctness as much as it can without knowing the T. However the MSVC is known for violating that requirement and not bothering to check anything thus allowing a complete nonsense in method definition which will only be discovered when the method is actually used.
  
  – r3mus n0x
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vesil, the nonsense example: godbolt.org/z/q0Xvvz :)
  
  – r3mus n0x
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So if the method promote() is virtual it will fail in compilation?
  
  – vesii
  3 hours ago
  
  

  
  
  
  

add a comment | 

7

Template class methods are not instantiated until they are used. Once you try calling promote() or even get its address like this &Val<int>::promote then you'll get an error.

From the C++ standard:

§ 17.8.1.10 An implementation shall not implicitly instantiate a function
template, a variable template, a member template, a non-virtual member
function, a member class, a static data member of a class template,
or a substatement of a constexpr if statement (9.4.1), unless such
instantiation is required.

share|improve this answer

edited 6 hours ago

alter igel

4,26511 gold badge1414 silver badges3333 bronze badges

answered 8 hours ago

r3mus n0xr3mus n0x

3,72211 gold badge33 silver badges2525 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thank you. Are there any checks at all that the compiler does on promote()? I guess syntax errors.
  
  – vesii
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vesil, AFAIK the compiler is obligated by the standard to parse the non-instantiated method definition and check it for correctness as much as it can without knowing the T. However the MSVC is known for violating that requirement and not bothering to check anything thus allowing a complete nonsense in method definition which will only be discovered when the method is actually used.
  
  – r3mus n0x
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @vesil, the nonsense example: godbolt.org/z/q0Xvvz :)
  
  – r3mus n0x
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So if the method promote() is virtual it will fail in compilation?
  
  – vesii
  3 hours ago
  
  

  
  
  
  

add a comment | 

Template class methods are not instantiated until they are used. Once you try calling promote() or even get its address like this &Val<int>::promote then you'll get an error.

From the C++ standard:

§ 17.8.1.10 An implementation shall not implicitly instantiate a function
template, a variable template, a member template, a non-virtual member
function, a member class, a static data member of a class template,
or a substatement of a constexpr if statement (9.4.1), unless such
instantiation is required.

share|improve this answer

edited 6 hours ago

alter igel

4,26511 gold badge1414 silver badges3333 bronze badges

answered 8 hours ago

r3mus n0xr3mus n0x

3,72211 gold badge33 silver badges2525 bronze badges

share|improve this answer

edited 6 hours ago

alter igel

4,26511 gold badge1414 silver badges3333 bronze badges

answered 8 hours ago

r3mus n0xr3mus n0x

3,72211 gold badge33 silver badges2525 bronze badges

edited 6 hours ago

alter igel

4,26511 gold badge1414 silver badges3333 bronze badges

edited 6 hours ago

alter igel

4,26511 gold badge1414 silver badges3333 bronze badges

answered 8 hours ago

r3mus n0xr3mus n0x

3,72211 gold badge33 silver badges2525 bronze badges

answered 8 hours ago

r3mus n0xr3mus n0x

3,72211 gold badge33 silver badges2525 bronze badges

5

Templates have always worked this way, principally to facilitate their use.

Because Val<int>(4).val(); doesn't call promote, that function is not compiled for your particular instantiation of that template so the compiler does not issue a diagnostic.

Many metaprogramming techniques depend on this behaviour.

share|improve this answer

answered 8 hours ago

BathshebaBathsheba

189k2727 gold badges282282 silver badges400400 bronze badges

add a comment | 

5

Templates have always worked this way, principally to facilitate their use.

Because Val<int>(4).val(); doesn't call promote, that function is not compiled for your particular instantiation of that template so the compiler does not issue a diagnostic.

Many metaprogramming techniques depend on this behaviour.

share|improve this answer

answered 8 hours ago

BathshebaBathsheba

189k2727 gold badges282282 silver badges400400 bronze badges

add a comment | 

Templates have always worked this way, principally to facilitate their use.

Because Val<int>(4).val(); doesn't call promote, that function is not compiled for your particular instantiation of that template so the compiler does not issue a diagnostic.

Many metaprogramming techniques depend on this behaviour.

share|improve this answer

answered 8 hours ago

BathshebaBathsheba

189k2727 gold badges282282 silver badges400400 bronze badges

share|improve this answer

answered 8 hours ago

BathshebaBathsheba

189k2727 gold badges282282 silver badges400400 bronze badges

answered 8 hours ago

BathshebaBathsheba

189k2727 gold badges282282 silver badges400400 bronze badges

answered 8 hours ago

BathshebaBathsheba

189k2727 gold badges282282 silver badges400400 bronze badges